Indebtedness - Additional Information (Details) - USD ($)	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Loan outstanding balance	$ 1,497,609,000	$ 1,294,945,000
Recourse debt
Debt Instrument [Line Items]
Loan outstanding balance	247,000,000	247,000,000
Non-recourse debt
Debt Instrument [Line Items]
Loan outstanding balance	1,250,609,000	1,047,945,000
Non-recourse debt | Aggregation facility, March 2023
Debt Instrument [Line Items]
Loan outstanding balance	313,800,000
Non-recourse debt | Aggregation facility, December 2021
Debt Instrument [Line Items]
Loan outstanding balance	195,400,000
Non-recourse debt | Line of credit | Aggregation facility, March 2023
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity	595,000,000
Non-recourse debt | Line of credit | Aggregation facility, December 2021
Debt Instrument [Line Items]
Loan outstanding balance	$ 161,600,000
Non-recourse debt | London Interbank Offered Rate (LIBOR) | Line of credit | Aggregation facility, March 2023
Debt Instrument [Line Items]
Basis spread on variable rate	2.50%
Facility available period	3 years
Debt instrument basis spread on variable rate, periodic increase	2.75%
Revolving line of credit facility available period, period increase	2 years
Bank line of credit | Recourse debt
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity	$ 250,000,000
Minimum unencumbered liquid assets to be maintained	$ 30,000,000
Interest coverage ratio	300.00%
Loan outstanding balance	$ 247,000,000	247,000,000
Bank line of credit | Recourse debt | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate	3.25%
Bank line of credit | Recourse debt | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate	2.25%
Bank line of credit | Recourse debt | Federal Funds Rate
Debt Instrument [Line Items]
Basis spread on variable rate	0.50%
Bank line of credit | Recourse debt | LIBOR Floor Rate
Debt Instrument [Line Items]
Basis spread on variable rate	1.00%
Term Loan | Non-recourse debt | Minimum | Aggregation facility, March 2023
Debt Instrument [Line Items]
Term loan prepayment penalty, percent	0.00%
Term Loan | Non-recourse debt | Maximum | Aggregation facility, March 2023
Debt Instrument [Line Items]
Term loan prepayment penalty, percent	1.00%
Term Loan | Non-recourse debt | London Interbank Offered Rate (LIBOR) | Aggregation facility, March 2023
Debt Instrument [Line Items]
Basis spread on variable rate	5.00%
Facility available period	3 years
Debt instrument basis spread on variable rate, periodic increase	6.50%
Term Loan Due in October 2024 | Non-recourse debt | Senior secured revolving letter of credit facility | Aggregation facility, October 2024
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity	$ 303,000,000
Loan outstanding balance	286,700,000
Term Loan Due in October 2024 | Non-recourse debt | Revolving debt | Aggregation facility, October 2024
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity	10,000,000
Term Loan Due in October 2024 | Non-recourse debt | Term Loan | Aggregation facility, October 2024
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity	$ 293,000,000
Term Loan A | Non-recourse debt | London Interbank Offered Rate (LIBOR) | Aggregation facility, October 2024
Debt Instrument [Line Items]
Basis spread on variable rate	2.75%
Term Loan A | Non-recourse debt | London Interbank Offered Rate (LIBOR) | Aggregation facility, December 2021
Debt Instrument [Line Items]
Basis spread on variable rate	2.75%
Debt instrument basis spread on variable rate, periodic increase	3.00%
Term Loan A | Non-recourse debt | Base Rate | Aggregation facility, October 2024
Debt Instrument [Line Items]
Basis spread on variable rate	1.75%
Term Loan B | Non-recourse debt | Minimum | Aggregation facility, October 2024
Debt Instrument [Line Items]
Term loan prepayment penalty, percent	0.00%
Term Loan B | Non-recourse debt | Maximum | Aggregation facility, October 2024
Debt Instrument [Line Items]
Term loan prepayment penalty, percent	5.00%
Term Loan B | Non-recourse debt | London Interbank Offered Rate (LIBOR) | Aggregation facility, December 2021
Debt Instrument [Line Items]
Basis spread on variable rate	5.00%
Term Loan B, Class A | Non-recourse debt | London Interbank Offered Rate (LIBOR) | Aggregation facility, October 2024
Debt Instrument [Line Items]
Basis spread on variable rate	7.03%
Term Loan B, Class B | Non-recourse debt | London Interbank Offered Rate (LIBOR) | Aggregation facility, October 2024
Debt Instrument [Line Items]
Basis spread on variable rate	5.00%
Basis spread on Federal Funds Rate	1.00%
Term Loan B, Class B | Non-recourse debt | Base Rate | Aggregation facility, October 2024
Debt Instrument [Line Items]
Basis spread on variable rate	4.00%
Basis spread on Federal Funds Rate	0.50%
Term Loan Due in April 2024 | Non-recourse debt | Senior secured revolving letter of credit facility | Aggregation facility, April 2024
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity	$ 202,000,000
Loan outstanding balance	188,100,000
Term Loan Due in April 2024 | Non-recourse debt | Delayed draw term loan | Aggregation facility, April 2024
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity	195,000,000
Term Loan Due in April 2024 | Non-recourse debt | Revolving debt | Aggregation facility, April 2024
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity	$ 7,000,000
Term Loan Due in April 2024 | Non-recourse debt | London Interbank Offered Rate (LIBOR) | Aggregation facility, April 2024
Debt Instrument [Line Items]
Basis spread on variable rate	2.25%
Facility available period	4 years
Term Loan Due in April 2024 | Non-recourse debt | Base Rate | Aggregation facility, April 2024
Debt Instrument [Line Items]
Basis spread on variable rate	1.25%
Term Loan Due in April 2024 | Non-recourse debt | Federal Funds Rate | Aggregation facility, April 2024
Debt Instrument [Line Items]
Basis spread on variable rate	0.50%
Term Loan Due in April 2024 | Non-recourse debt | LIBOR Floor Rate | Aggregation facility, April 2024
Debt Instrument [Line Items]
Basis spread on variable rate	1.00%
Senior debt | Non-recourse debt
Debt Instrument [Line Items]
Loan outstanding balance	$ 1,003,688,000	808,455,000
Senior debt | Non-recourse debt | London Interbank Offered Rate (LIBOR) | Revolving loan facility, September 2020
Debt Instrument [Line Items]
Basis spread on variable rate	2.75%
Bank term loan due September 2020 | Non-recourse debt | Revolving loan facility, September 2020
Debt Instrument [Line Items]
Loan outstanding balance	$ 154,400,000
Subordinated debt | Non-recourse debt
Debt Instrument [Line Items]
Loan outstanding balance	$ 153,674,000	111,488,000
Subordinated debt | Non-recourse debt | London Interbank Offered Rate (LIBOR) | Revolving loan facility, September 2020
Debt Instrument [Line Items]
Basis spread on variable rate	5.50%
Banke Term Loan Due April 2022 | Non-recourse debt
Debt Instrument [Line Items]
Loan outstanding balance	$ 22,900,000
Bank term loans due September 2022, Tranche One | Non-recourse debt
Debt Instrument [Line Items]
Basis spread on variable rate	3.00%
Loan outstanding balance	$ 11,600,000
Bank term loans due September 2022, Tranche Two | Non-recourse debt
Debt Instrument [Line Items]
Basis spread on variable rate	2.25%
Loan outstanding balance	$ 18,300,000
Securitization loans
Debt Instrument [Line Items]
Secured borrowings assets carrying amount	168,500,000	172,800,000
Securitization loans | Non-recourse debt
Debt Instrument [Line Items]
Loan outstanding balance	$ 93,200,000
SREC Loans | Non-recourse debt
Debt Instrument [Line Items]
Loan outstanding balance		$ 32,181,000