Indebtedness - Additional Information (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Loan outstanding balance		$ 1,748,922,000	$ 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		1,501,922,000	1,047,945,000
Senior secured revolving letter of credit facility
Debt Instrument [Line Items]
Loan outstanding balance		$ 282,700,000
LIBOR | Line of Credit | Non Recourse Debt
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		2.50%
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		$ 316,200,000
Bank line of credit
Debt Instrument [Line Items]
Minimum unencumbered liquid assets to be maintained		25,000,000
Debt covenant, liability maintenance		30,000,000
Bank line of credit | Recourse Debt
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity		$ 250,000,000
Interest coverage ratio		300.00%
Loan outstanding balance		$ 247,000,000	247,000,000
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%
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		184,400,000
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		2.25%
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		1.25%
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 | Aggregation Facility, October 2024 | LIBOR | Non Recourse Debt
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate, federal funds rate		1.00%
Term Loan Due in December 2020 | Non Recourse Debt
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity		$ 595,000,000
Term Loan B | Non Recourse Debt
Debt Instrument [Line Items]
Prepayment penalty		$ 0
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]
Prepayment penalty percent		0.00%
Term Loan B | Maximum | Non Recourse Debt
Debt Instrument [Line Items]
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 | LIBOR | Non Recourse Debt
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		5.00%
Facility available period	3 years
Debt instrument, variable rate periodic increase		6.50%
Term Loan | Minimum | Non Recourse Debt
Debt Instrument [Line Items]
Prepayment penalty percent		0.00%
Term Loan | Maximum | Non Recourse Debt
Debt Instrument [Line Items]
Prepayment penalty percent		1.00%
Term Loan A | LIBOR | Non Recourse Debt
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		2.75%
Term Loan A | Base Rate | Non Recourse Debt
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		1.75%
Bank Loan Due September 2020 | Non Recourse Debt
Debt Instrument [Line Items]
Loan outstanding balance		$ 162,400,000
Bank Loan Due September 2020 | LIBOR | Non Recourse Debt
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		2.75%
Subordinated debt | Non Recourse Debt
Debt Instrument [Line Items]
Loan outstanding balance		$ 273,337,000	111,488,000
Subordinated debt | Revolving Loan Facility, September 2020 | LIBOR | Non Recourse Debt
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		5.50%
Bank term loan due April 2022 | Non Recourse Debt
Debt Instrument [Line Items]
Loan outstanding balance		$ 20,800,000
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		$ 16,800,000
Bank term loan due January 2030 | Non Recourse Debt | Subsidiary three
Debt Instrument [Line Items]
Loan outstanding balance		118,600,000
Securitization loans | Subsidiary one
Debt Instrument [Line Items]
Secured borrowing, carrying amount		$ 164,700,000	172,800,000
Debt instrument discount rate		0.08%
Securitization loans | Subsidiary two
Debt Instrument [Line Items]
Secured borrowing, carrying amount		$ 72,000,000
Debt instrument discount rate		1.47%
Securitization loans | Non Recourse Debt | Subsidiary one
Debt Instrument [Line Items]
Loan outstanding balance		$ 90,200,000
Securitization loans | Non Recourse Debt | Subsidiary two
Debt Instrument [Line Items]
Loan outstanding balance		$ 309,800,000
SREC Loans | Non Recourse Debt
Debt Instrument [Line Items]
Loan outstanding balance			$ 32,181,000