Annual report pursuant to Section 13 and 15(d)

Indebtedness - Additional Information (Details)

v3.8.0.1
Indebtedness - Additional Information (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Mar. 31, 2018
Debt Instrument [Line Items]        
Recourse debt, net of current portion $ 247,000,000 $ 244,000,000    
Loan outstanding balance 1,294,945,000 898,023,000    
Recourse Debt        
Debt Instrument [Line Items]        
Loan outstanding balance 247,000,000 244,000,000    
Repayment of non-recourse debt 167,400,000 411,400,000 $ 348,224,000  
Non Recourse Debt        
Debt Instrument [Line Items]        
Loan outstanding balance 1,047,945,000 654,023,000    
Repayment of non-recourse debt 362,763,000 23,076,000 $ 11,774,000  
Senior secured revolving letter of credit facility        
Debt Instrument [Line Items]        
Loan outstanding balance $ 280,800,000      
LIBOR | Line of Credit | Non Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 2.50%      
Revolving line of credit facility available period 3 years      
Debt instrument, variable rate periodic increase 2.75%      
Revolving line of credit facility available period, period increase 2 years      
Aggregation facility | Line of Credit | Non Recourse Debt        
Debt Instrument [Line Items]        
Loan outstanding balance $ 150,700,000      
Bank line of credit        
Debt Instrument [Line Items]        
Minimum unencumbered liquid assets to be maintained $ 25,000,000      
Interest coverage ratio 200.00%      
Recourse debt, net of current portion $ 247,000,000      
Bank line of credit | Recourse Debt        
Debt Instrument [Line Items]        
Line of credit, maximum borrowing capacity 250,000,000      
Loan outstanding balance $ 247,000,000 244,000,000    
Bank line of credit | LIBOR | Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 3.25%      
Bank line of credit | Base Rate | Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 2.25%      
Bank line of credit | Federal Funds Rate | Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 0.50%      
Bank line of credit | Prime Rate | Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 1.00%      
Bank line of credit | Credit facility agreements with syndicate of banks | Minimum        
Debt Instrument [Line Items]        
Interest coverage ratio 200.00%      
Bank line of credit | Credit facility agreements with syndicate of banks | Maximum        
Debt Instrument [Line Items]        
Interest coverage ratio 300.00%      
Term Loan Due in October 2024 | Non Recourse Debt        
Debt Instrument [Line Items]        
Line of credit, maximum borrowing capacity $ 303,000,000      
Term Loan Due in October 2024 | Term Loan | Non Recourse Debt        
Debt Instrument [Line Items]        
Line of credit, maximum borrowing capacity 293,000,000      
Term Loan Due in October 2024 | Letter of Credit | Non Recourse Debt        
Debt Instrument [Line Items]        
Line of credit, maximum borrowing capacity 10,000,000      
Term Loan Due in April 2024 | Non Recourse Debt        
Debt Instrument [Line Items]        
Line of credit, maximum borrowing capacity 202,000,000      
Loan outstanding balance 157,000,000      
Term Loan Due in April 2024 | Term Loan | Non Recourse Debt        
Debt Instrument [Line Items]        
Line of credit, maximum borrowing capacity 195,000,000      
Term Loan Due in April 2024 | Letter of Credit | Non Recourse Debt        
Debt Instrument [Line Items]        
Line of credit, maximum borrowing capacity $ 7,000,000      
Term Loan Due in April 2024 | LIBOR | Non Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 2.75%      
Revolving line of credit facility available period 4 years      
Term Loan Due in April 2024 | Base Rate | Non Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 1.75%      
Term Loan Due in April 2024 | Federal Funds Rate | Non Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 0.50%      
Term Loan Due in April 2024 | LIBOR Floor Rate | Non Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 1.00%      
Term Loan Due in December 2020 | Non Recourse Debt        
Debt Instrument [Line Items]        
Line of credit, maximum borrowing capacity $ 340,000,000      
Term Loan B | LIBOR | Non Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 5.00%      
Term Loan B | Base Rate | Non Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 4.00%      
Term Loan B | Federal Funds Rate | Non Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 0.50%      
Term Loan B | LIBOR Floor Rate | Non Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 1.00%      
Term Loan B | Minimum | Non Recourse Debt        
Debt Instrument [Line Items]        
Term loan prepayment penalty, percent 0.00%      
Term Loan B | Maximum | Non Recourse Debt        
Debt Instrument [Line Items]        
Term loan prepayment penalty, percent 5.00%      
Term Loan B, Class A | LIBOR | Non Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 7.03%      
Term Loan B, Class B | LIBOR | Non Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 5.00%      
Term Loan | Non Recourse Debt        
Debt Instrument [Line Items]        
Loan outstanding balance $ 163,700,000      
Term Loan | LIBOR | Non Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 5.00%      
Revolving line of credit facility available period 3 years      
Debt instrument, variable rate periodic increase 6.50%      
Term Loan | Minimum | Non Recourse Debt        
Debt Instrument [Line Items]        
Term loan prepayment penalty, percent 0.00%      
Term Loan | Maximum | Non Recourse Debt        
Debt Instrument [Line Items]        
Term loan prepayment penalty, percent 1.00%      
Term Loan A | Non Recourse Debt        
Debt Instrument [Line Items]        
Line of credit, maximum borrowing capacity $ 195,400,000      
Term Loan A | LIBOR | Non Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 2.75%      
Debt instrument, variable rate periodic increase 3.00%      
Bank Loan Due September 2020 | Non Recourse Debt        
Debt Instrument [Line Items]        
Loan outstanding balance $ 111,400,000      
Bank Loan Due September 2020 | LIBOR | Non Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 2.75%      
Debt instrument, variable rate periodic increase 5.50%      
Bank term loan due April 2022 | Non Recourse Debt        
Debt Instrument [Line Items]        
Loan outstanding balance $ 24,500,000      
Bank term loan due September 2022, tranche one | Non Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 3.00%      
Loan outstanding balance $ 12,300,000      
Bank term loan due September 2022, tranche two | Non Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 2.25%      
Loan outstanding balance $ 19,700,000      
Securitization loans        
Debt Instrument [Line Items]        
Secured borrowing, carrying amount $ 172,800,000 181,800,000    
Debt instrument discount rate 0.08%      
Securitization loans | Non Recourse Debt        
Debt Instrument [Line Items]        
Loan outstanding balance $ 95,800,000      
SREC Loans | Non Recourse Debt        
Debt Instrument [Line Items]        
Loan outstanding balance $ 32,181,000 $ 31,088,000    
Interest Rate (1) 7.28% 6.88%    
Note Payable | Non Recourse Debt        
Debt Instrument [Line Items]        
Loan outstanding balance   $ 36,232,000    
Interest Rate (1)   12.00%    
Repayment of non-recourse debt $ 40,900,000      
Gain (loss) on extinguishment of debt $ (600,000)      
Contracted SRECs | SREC Loans | LIBOR | Non Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 5.50%      
Uncontracted SRECs | SREC Loans | LIBOR | Non Recourse Debt        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate 9.00%      
Scenario, forecast | Bank line of credit | Credit facility agreements with syndicate of banks        
Debt Instrument [Line Items]        
Minimum unencumbered liquid assets to be maintained       $ 30,000,000