Securitizations and Variable Interest Entities - Fair Value Sensitivity (Details) - Asset-backed securities - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
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	$ 37,020	$ 0
Expected weighte-average life (in years)	1 year
Discount Rates, 100 Basis Point Increase	$ (326)
Discount Rates, 200 Basis Point Increase	(644)
Expected credit loss rates on underlying loans, 10% adverse change	(1)
Expected credit loss rates on underlying loans, 20% adverse change	(2)
Expected prepayment rates, 10% adverse change	(1)
Expected prepayment rates, 20% adverse change	(3)
Subordinated Debt Obligations [Member]
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	$ 8,236	0
Expected weighte-average life (in years)	1 year 6 months
Discount Rates, 100 Basis Point Increase	$ (105)
Discount Rates, 200 Basis Point Increase	(208)
Expected credit loss rates on underlying loans, 10% adverse change	(1,060)
Expected credit loss rates on underlying loans, 20% adverse change	(2,118)
Expected prepayment rates, 10% adverse change	(265)
Expected prepayment rates, 20% adverse change	(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	$ 1,793	$ 0
Expected weighte-average life (in years)	1 year 5 months
Discount Rates, 100 Basis Point Increase	$ (41)
Discount Rates, 200 Basis Point Increase	(76)
Expected credit loss rates on underlying loans, 10% adverse change	(15)
Expected credit loss rates on underlying loans, 20% adverse change	(25)
Expected prepayment rates, 10% adverse change	(21)
Expected prepayment rates, 20% adverse change	$ (42)