Quarterly report pursuant to Section 13 or 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
Sep. 30, 2018
Dec. 31, 2017
Carrying Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value $ 1,564,598 $ 1,294,945
Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 1,559,791 1,294,973
Bank line of credit | Carrying Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 247,000 247,000
Bank line of credit | Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 247,000 247,000
Senior debt | Carrying Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 1,067,947 808,455
Senior debt | Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 1,067,529 807,698
Subordinated debt | Carrying Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 157,943 111,488
Subordinated debt | Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 156,257 111,095
Securitization debt | Carrying Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 91,708 95,821
Securitization debt | Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 89,005 96,999
SREC Loans | Carrying Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value 0 32,181
SREC Loans | Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, fair value $ 0 $ 32,181