Securities Available for Sale - Amortized cost/fair value (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 150,264	$ 117,545
Gross Unrealized Gains	116	86
Gross Unrealized Losses	(576)	(58)
Fair Value	149,804	117,573
Asset-backed senior securities related to Company-sponsored securitizations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	65,044	36,953
Gross Unrealized Gains	57	73
Gross Unrealized Losses	(114)	(6)
Fair Value	64,987	37,020
Asset-backed subordinated residual certificates related to Company-sponsored securitizations and CLUB Certificate transactions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	29,127	10,058
Gross Unrealized Gains	56	11
Gross Unrealized Losses	(456)	(40)
Fair Value	28,727	10,029
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	17,747	16,268
Gross Unrealized Gains	3	1
Gross Unrealized Losses	(4)	(11)
Fair Value	17,746	16,258
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,796	24,758
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	15,796	24,758
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,077	14,843
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(2)	(1)
Fair Value	13,075	14,843
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,473	14,665
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	9,473	14,665
Asset-backed Securities, Securitized Loans and Receivables, Subject to Restrictions on Transfer [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	92,000	$ 45,300
Asset-backed senior securities related to Company-sponsored securitizations (1)
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 93,714

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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