Fair Value of Assets and Liabilities - Sensitivity of Fair Value of Loans Invested in by Company (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2019	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale at fair value	$ 270,927	$ 170,469
Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale at fair value	110,796	60,279
Asset-backed Securities Related to Structured Program Transactions, Senior Securities | Fair Value, Measurements, Recurring | Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale at fair value	109,339	56,489
Discount rates, impact of 100 basis point increase	(1,050)	(526)
Discount rates, impact of 200 basis point increase	(2,076)	(1,032)
Expected credit loss rates on underlying loans, 10% adverse change	0	0
Expected credit loss on underlying loans, 20% adverse change	0	0
Expected prepayment rates, 10% adverse change	0	0
Expected prepayment rates, 20% adverse change	$ 0	$ 0
Asset-backed Securities Related to Structured Program Transactions, Senior Securities | Measurement Input, Expected Term | Fair Value, Measurements, Recurring | Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Expected weighted-average life (in years)	1 year 1 month 6 days	1 year
Asset-backed Securities Related to Structured Program Transactions, Subordinated Residual Certificates | Fair Value, Measurements, Recurring | Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale at fair value	$ 21,090	$ 11,849
Discount rates, impact of 100 basis point increase	(300)	(149)
Discount rates, impact of 200 basis point increase	(513)	(293)
Expected credit loss rates on underlying loans, 10% adverse change	(2,162)	(1,573)
Expected credit loss on underlying loans, 20% adverse change	(4,273)	(3,159)
Expected prepayment rates, 10% adverse change	(814)	(786)
Expected prepayment rates, 20% adverse change	$ (1,495)	$ (1,599)
Asset-backed Securities Related to Structured Program Transactions, Subordinated Residual Certificates | Measurement Input, Expected Term | Fair Value, Measurements, Recurring | Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Expected weighted-average life (in years)	1 year 4 months 24 days	1 year 3 months 18 days
Asset-backed Securities Related to Structured Program Transactions, CLUB Transactions | Fair Value, Measurements, Recurring | Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale at fair value	$ 89,706	$ 48,430
Discount rates, impact of 100 basis point increase	(823)	(472)
Discount rates, impact of 200 basis point increase	(1,627)	(932)
Expected credit loss rates on underlying loans, 10% adverse change	(2,163)	(1,070)
Expected credit loss on underlying loans, 20% adverse change	(4,311)	(2,112)
Expected prepayment rates, 10% adverse change	(654)	(291)
Expected prepayment rates, 20% adverse change	$ (1,279)	$ (562)
Asset-backed Securities Related to Structured Program Transactions, CLUB Transactions | Measurement Input, Expected Term | Fair Value, Measurements, Recurring | Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Expected weighted-average life (in years)	1 year 1 month 6 days	1 year 2 months 12 days
Loans Invested in by Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of loans invested in by the Company	$ 766,048	$ 842,604		$ 597,055
Loans Invested in by Company | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of loans invested in by the Company	766,048	842,604	$ 766,000
Discount rate, 100 basis point increase	(9,806)	(10,487)
Discount rate, 200 basis point increase	(19,410)	(20,720)
Expected credit loss rates on underlying loans, 10% adverse change	(9,558)	(11,304)
Expected credit loss rates on underlying loans, 20% adverse change	(19,136)	(22,504)
Expected prepayment rates, 10% adverse change	(2,429)	(2,422)
Expected prepayment rates, 20% adverse change	$ (4,740)	$ (4,785)
Loans Invested in by Company | Measurement Input, Expected Term | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Expected weighted-average life (in years)	1 year 6 months	1 year 4 months 24 days