Securities Available for Sale - Amortized cost/fair value (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 128,374	$ 117,545
Gross Unrealized Gains	181	86
Gross Unrealized Losses	(131)	(58)
Fair Value	128,424	117,573
Level 3 Inputs
Schedule of Available-for-sale Securities [Line Items]
Fair Value	20,129	10,029
Asset-backed senior securities related to Company-sponsored securitizations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	45,875	36,953
Gross Unrealized Gains	54	73
Gross Unrealized Losses	(48)	(6)
Fair Value	45,881	37,020
Asset-backed senior securities related to Company-sponsored securitizations | Level 3 Inputs
Schedule of Available-for-sale Securities [Line Items]
Fair Value	0	0
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,303	24,758
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	20,303	24,758
Certificates of deposit | Level 3 Inputs
Schedule of Available-for-sale Securities [Line Items]
Fair Value	0	0
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19,853	16,268
Gross Unrealized Gains	2	1
Gross Unrealized Losses	(15)	(11)
Fair Value	19,840	16,258
Corporate debt securities | Level 3 Inputs
Schedule of Available-for-sale Securities [Line Items]
Fair Value	0	0
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,588	14,843
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(2)	(1)
Fair Value	14,586	14,843
Asset-backed securities | Level 3 Inputs
Schedule of Available-for-sale Securities [Line Items]
Fair Value	0	0
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,685	14,665
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	7,685	14,665
Commercial paper | Level 3 Inputs
Schedule of Available-for-sale Securities [Line Items]
Fair Value	0	0
Asset-backed subordinated residual certificates related to Company-sponsored securitizations and CLUB Certificate transactions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,070	10,058
Gross Unrealized Gains	125	11
Gross Unrealized Losses	(66)	(40)
Fair Value	20,129	10,029
Asset-backed subordinated residual certificates related to Company-sponsored securitizations and CLUB Certificate transactions | Level 3 Inputs
Schedule of Available-for-sale Securities [Line Items]
Fair Value	20,129	10,029
Asset-backed Securities, Securitized Loans and Receivables, Subject to Restrictions on Transfer [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	64,200	45,300
Asset-backed senior securities related to Company-sponsored securitizations (1)
Schedule of Available-for-sale Securities [Line Items]
Fair Value	66,010
Asset-backed senior securities related to Company-sponsored securitizations (1) | Level 3 Inputs | Subordinated residual certificates
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 20,100	$ 10,000