Securities Available for Sale - Continuous loss (Details) $ in Thousands	Mar. 31, 2018 USD ($) position	Dec. 31, 2017 USD ($) position
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 months	$ 59,143	$ 45,303
Unrealized Losses, Less than 12 months	(131)	(58)
Fair Value, 12 months or longer	0	0
Unrealized Losses, 12 months or longer	0	0
Fair Value	59,143	45,303
Unrealized Losses	$ (131)	$ (58)
Number of positions with unrealized losses | position	30	24
Asset-backed senior securities related to Company-sponsored securitizations (1)
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 months	$ 42,018	$ 26,534
Unrealized Losses, Less than 12 months	(114)	(46)
Fair Value, 12 months or longer	0	0
Unrealized Losses, 12 months or longer	0	0
Fair Value	42,018	26,534
Unrealized Losses	(114)	(46)
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 months	15,377	14,368
Unrealized Losses, Less than 12 months	(15)	(11)
Fair Value, 12 months or longer	0	0
Unrealized Losses, 12 months or longer	0	0
Fair Value	15,377	14,368
Unrealized Losses	(15)	(11)
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 months	1,748	4,401
Unrealized Losses, Less than 12 months	(2)	(1)
Fair Value, 12 months or longer	0	0
Unrealized Losses, 12 months or longer	0	0
Fair Value	1,748	4,401
Unrealized Losses	$ (2)	$ (1)