Securitization and Variable Interest Entities - Fair Value Sensitivity (Details) - Asset-backed securities - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Senior securities
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Fair value of interests held	$ 45,881	$ 37,020
Expected weighted-average life (in years)	8 months	1 year
Discount rates, 100 basis point increase	$ (442)	$ (326)
Discount rates, 200 basis point increase	(875)	(644)
Expected credit loss on rates on underlying loans, 10% adverse change	0	(1)
Expected credit loss on rates on underlying loans, 20% adverse change	0	(2)
Expected prepayment rates, 10% adverse change	0	(1)
Expected prepayment rates, 20% adverse change	0	(3)
Subordinated residual certificates
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Fair value of interests held	$ 10,351	$ 8,236
Expected weighted-average life (in years)	1 year 7 months	1 year 6 months
Discount rates, 100 basis point increase	$ (127)	$ (105)
Discount rates, 200 basis point increase	(250)	(208)
Expected credit loss on rates on underlying loans, 10% adverse change	(1,454)	(1,060)
Expected credit loss on rates on underlying loans, 20% adverse change	(2,875)	(2,118)
Expected prepayment rates, 10% adverse change	(507)	(265)
Expected prepayment rates, 20% adverse change	(988)	(513)
CLUB Certificates
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Fair value of interests held	$ 9,778	$ 1,793
Expected weighted-average life (in years)	1 year 3 months	1 year 5 months
Discount rates, 100 basis point increase	$ (119)	$ (41)
Discount rates, 200 basis point increase	(236)	(76)
Expected credit loss on rates on underlying loans, 10% adverse change	(171)	(15)
Expected credit loss on rates on underlying loans, 20% adverse change	(308)	(25)
Expected prepayment rates, 10% adverse change	(77)	(21)
Expected prepayment rates, 20% adverse change	$ (120)	$ (42)