Fair Value of Assets and Liabilities - Sensitivity of Fair Value of Loans Invested in by the Company, Asset-backed Securities Related to Structured Program Transactions and Servicing Assets (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2021	Dec. 31, 2020	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for sale at fair value	$ 180,061	$ 121,902
Loans held for sale by the Company at fair value		0
Securities available for sale at fair value	277,505	142,226
Servicing assets	58,728	56,347
Asset-backed subordinated securities related to Company-sponsored Structured Program transactions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale at fair value		16,515
Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for sale at fair value	180,061	121,902
Securities available for sale at fair value	45,918	66,654
Servicing assets	58,728	56,347
Level 3 Inputs | Asset-backed Securities Related to Structured Program Transactions, Senior Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale at fair value	49,555
Level 3 Inputs | Asset-backed Securities Related to Structured Program Transactions, Subordinated Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale at fair value	13,761
Level 3 Inputs | Asset-backed Securities Related to Structured Program Transactions, CLUB Transactions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale at fair value	32,157
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing assets	58,728	56,347	$ 54,113	$ 65,515	$ 92,825	$ 89,680
Fair Value, Measurements, Recurring | Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing assets	58,728	56,347
Fair Value, Measurements, Recurring | Level 3 Inputs | Servicing Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rates, impact of 100 basis point increase	(484)	(455)
Discount rates, impact of 200 basis point increase	(967)	(911)
Expected credit loss on rates on underlying loans, 10% adverse change	(270)	(346)
Expected credit loss on rates on underlying loans, 20% adverse change	(540)	(691)
Expected prepayment rates, 10% adverse change	(1,452)	(1,596)
Expected prepayment rates, 20% adverse change	(2,903)	(3,192)
Fair Value, Measurements, Recurring | Level 3 Inputs | Asset-backed Securities Related to Structured Program Transactions, Senior Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale at fair value		75,372
Discount rates, impact of 100 basis point increase	(339)	(579)
Discount rates, impact of 200 basis point increase	(671)	(1,145)
Expected credit loss on rates on underlying loans, 10% adverse change	0	0
Expected credit loss on rates on underlying loans, 20% adverse change	0	0
Expected prepayment rates, 10% adverse change	0	0
Expected prepayment rates, 20% adverse change	0	0
Fair Value, Measurements, Recurring | Level 3 Inputs | Asset-backed Securities Related to Structured Program Transactions, Subordinated Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale at fair value		16,515
Discount rates, impact of 100 basis point increase	(127)	(161)
Discount rates, impact of 200 basis point increase	(271)	(343)
Expected credit loss on rates on underlying loans, 10% adverse change	(550)	(1,831)
Expected credit loss on rates on underlying loans, 20% adverse change	(2,035)	(3,718)
Expected prepayment rates, 10% adverse change	(551)	(791)
Expected prepayment rates, 20% adverse change	(1,289)	(1,736)
Fair Value, Measurements, Recurring | Level 3 Inputs | Asset-backed Securities Related to Structured Program Transactions, CLUB Transactions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale at fair value		50,139
Discount rates, impact of 100 basis point increase	(249)	(405)
Discount rates, impact of 200 basis point increase	(492)	(800)
Expected credit loss on rates on underlying loans, 10% adverse change	(780)	(1,528)
Expected credit loss on rates on underlying loans, 20% adverse change	(1,601)	(3,095)
Expected prepayment rates, 10% adverse change	(430)	(659)
Expected prepayment rates, 20% adverse change	$ (895)	$ (1,343)
Fair Value, Measurements, Recurring | Measurement Input, Expected Term | Level 3 Inputs | Asset-backed Securities Related to Structured Program Transactions, Senior Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted-average estimated life of contractual repurchase agreements	9 months 18 days	10 months 24 days
Fair Value, Measurements, Recurring | Measurement Input, Expected Term | Level 3 Inputs | Asset-backed Securities Related to Structured Program Transactions, Subordinated Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted-average estimated life of contractual repurchase agreements	1 year 3 months 18 days	1 year 4 months 24 days
Fair Value, Measurements, Recurring | Measurement Input, Expected Term | Level 3 Inputs | Asset-backed Securities Related to Structured Program Transactions, CLUB Transactions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted-average estimated life of contractual repurchase agreements	10 months 24 days	10 months 24 days
Loans Held For Sale | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for sale at fair value		$ 121,902
Discount rates, impact of 100 basis point increase	$ (1,886)	(1,151)
Discount rates, impact of 200 basis point increase	(3,741)	(2,282)
Expected credit loss rates on underlying loans, 10% adverse change	(2,204)	(1,099)
Expected credit loss rates on underlying loans, 20% adverse change	(4,479)	(2,220)
Expected prepayment rates, 10% adverse change	(835)	(273)
Expected prepayment rates, 20% adverse change	$ (1,725)	$ (556)
Loans Held For Sale | Fair Value, Measurements, Recurring | Measurement Input, Expected Term
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Expected weighted-average life (in years)	1 year 2 months 12 days	1 year 1 month 6 days
Loans Invested in by Company | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for sale by the Company at fair value	$ 34,523	$ 49,954
Discount rates, impact of 100 basis point increase	(316)	(541)
Discount rates, impact of 200 basis point increase	(628)	(1,073)
Expected credit loss rates on underlying loans, 10% adverse change	(334)	(640)
Expected credit loss rates on underlying loans, 20% adverse change	(680)	(1,295)
Expected prepayment rates, 10% adverse change	(164)	(181)
Expected prepayment rates, 20% adverse change	$ (339)	$ (368)
Loans Invested in by Company | Fair Value, Measurements, Recurring | Measurement Input, Expected Term
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Expected weighted-average life (in years)	1 year	1 year 1 month 6 days