Securities Available for Sale - Amortized cost/fair value (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 219,152	$ 287,293
Gross Unrealized Gains	202	91
Gross Unrealized Losses	(69)	(247)
Fair Value	219,285	287,137
Level 3 Inputs
Schedule of Available-for-sale Securities [Line Items]
Fair Value	5,402	0
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	123,007	181,359
Gross Unrealized Gains	176	63
Gross Unrealized Losses	(30)	(199)
Fair Value	123,153	181,223
Corporate debt securities | Level 3 Inputs
Schedule of Available-for-sale Securities [Line Items]
Fair Value	0	0
Asset-backed securities related to Company- sponsored securitization transactions (1)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	33,293
Gross Unrealized Gains	24
Gross Unrealized Losses	(32)
Fair Value	33,285
Asset-backed securities related to Company- sponsored securitization transactions (1) | Level 3 Inputs
Schedule of Available-for-sale Securities [Line Items]
Fair Value	5,402
Asset-backed securities related to Company- sponsored securitization transactions (1) | Level 3 Inputs | Subordinated residual certificates
Schedule of Available-for-sale Securities [Line Items]
Fair Value	5,400
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	25,920	27,501
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	25,920	27,501
Certificates of deposit | 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	15,454	25,369
Gross Unrealized Gains	0	4
Gross Unrealized Losses	(3)	(9)
Fair Value	15,451	25,364
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	9,981	20,164
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	9,981	20,164
Commercial paper | Level 3 Inputs
Schedule of Available-for-sale Securities [Line Items]
Fair Value	0	0
U.S. agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,000	19,602
Gross Unrealized Gains	1	21
Gross Unrealized Losses	0	0
Fair Value	5,001	19,623
U.S. agency securities | Level 3 Inputs
Schedule of Available-for-sale Securities [Line Items]
Fair Value	0	0
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,496	2,493
Gross Unrealized Gains	0	3
Gross Unrealized Losses	(4)	0
Fair Value	2,492	2,496
U.S. Treasury securities | Level 3 Inputs
Schedule of Available-for-sale Securities [Line Items]
Fair Value	0	0
Other securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,001	10,805
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	(39)
Fair Value	4,002	10,766
Other securities | Level 3 Inputs
Schedule of Available-for-sale Securities [Line Items]
Fair Value	0	$ 0
Asset-backed securities related to Company-sponsored securitization transactions subject to restrictions on transfer
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 32,500