Securities Available for Sale - Amortized cost/fair value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 165,128	$ 117,545
Gross Unrealized Gains	501	86
Gross Unrealized Losses	(187)	(58)
Fair Value	165,442	117,573
Securitized asset-backed senior securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	67,058	36,953
Gross Unrealized Gains	55	73
Gross Unrealized Losses	(58)	(6)
Fair Value	67,055	37,020
Securities pledged as collateral at fair value	24,700
CLUB Certificate asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	29,004	1,796
Gross Unrealized Gains	152	11
Gross Unrealized Losses	(110)	(14)
Fair Value	29,046	1,793
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	22,446	24,758
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	22,446	24,758
Securitized asset-backed subordinated residual certificates
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	13,302	8,262
Gross Unrealized Gains	291	0
Gross Unrealized Losses	(15)	(26)
Fair Value	13,578	8,236
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	12,037	14,843
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(1)	(1)
Fair Value	12,036	14,843
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	11,690	16,268
Gross Unrealized Gains	3	1
Gross Unrealized Losses	(3)	(11)
Fair Value	11,690	16,258
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,093	14,665
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	9,093	14,665
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	498
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	498
Asset-backed securities, securitized loans and receivables, subject to restrictions on transfer
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 108,000	$ 45,300

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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