Annual report pursuant to Section 13 and 15(d)

Fair Value Measurement - Schedule of Carrying Values and Fair Values of Debt Instruments (Details)

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Fair Value Measurement - Schedule of Carrying Values and Fair Values of Debt Instruments (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Carrying Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value $ 1,294,945 $ 898,023
Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 1,294,975 897,996
Bank line of credit | Carrying Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 247,000 244,000
Bank line of credit | Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 247,000 244,000
Senior debt | Carrying Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 808,455 442,420
Senior debt | Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 807,698 441,657
Subordinated debt | Carrying Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 111,488 42,985
Subordinated debt | Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 111,095 42,986
Securitization debt | Carrying Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 95,821 101,298
Securitization debt | Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 96,999 102,869
Note Payable | Carrying Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 0 36,232
Note Payable | Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 0 35,396
SREC Loans | Carrying Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 32,181 31,088
SREC Loans | Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value $ 32,183 $ 31,088