Securities Available for Sale - Unrealized losses (Details) $ in Thousands	Dec. 31, 2017 USD ($) position	Dec. 31, 2016 USD ($) position
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, fair value	$ 45,303	$ 121,290
Less than 12 months, unrealized loss	(58)	(196)
12 months or longer, fair value	0	17,518
12 months or longer, unrealized loss	0	(51)
Fair Value	45,303	138,808
Unrealized Losses	$ (58)	$ (247)
Number of positions with unrealized loss | position	24	72
Asset-backed securities related to Company-sponsored securitizations and CLUB Certificate transactions
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, fair value	$ 26,534
Less than 12 months, unrealized loss	(46)
12 months or longer, fair value	0
12 months or longer, unrealized loss	0
Fair Value	26,534
Unrealized Losses	(46)
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, fair value	14,368	$ 107,862
Less than 12 months, unrealized loss	(11)	(185)
12 months or longer, fair value	0	11,682
12 months or longer, unrealized loss	0	(14)
Fair Value	14,368	119,544
Unrealized Losses	(11)	(199)
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, fair value	4,401	6,628
Less than 12 months, unrealized loss	(1)	(8)
12 months or longer, fair value	0	1,870
12 months or longer, unrealized loss	0	(1)
Fair Value	4,401	8,498
Unrealized Losses	$ (1)	(9)
Other securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, fair value		6,800
Less than 12 months, unrealized loss		(3)
12 months or longer, fair value		3,966
12 months or longer, unrealized loss		(36)
Fair Value		10,766
Unrealized Losses		$ (39)