Securities Available for Sale - Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair value
Within 1 year	$ 70,524
After 1 year through 5 years	0
After 5 years through 10 years	45,256
After 10 years	1,793
Total	117,573	$ 287,137
Amortized cost
Within 1 year	70,534
After 1 year through 5 years	0
After 5 years through 10 years	45,215
After 10 years	1,796
Total	117,545
Asset-backed securities related to Company-sponsored securitizations and CLUB Certificate transactions
Fair value
Within 1 year	0
After 1 year through 5 years	0
After 5 years through 10 years	45,256
After 10 years	1,793
Total	47,049
Certificates of deposit
Fair value
Within 1 year	24,758
After 1 year through 5 years	0
After 5 years through 10 years	0
After 10 years	0
Total	24,758	27,501
Corporate debt securities
Fair value
Within 1 year	16,258
After 1 year through 5 years	0
After 5 years through 10 years	0
After 10 years	0
Total	16,258	181,223
Asset-backed securities
Fair value
Within 1 year	14,843
After 1 year through 5 years	0
After 5 years through 10 years	0
After 10 years	0
Total	14,843	25,364
Commercial paper
Fair value
Within 1 year	14,665
After 1 year through 5 years	0
After 5 years through 10 years	0
After 10 years	0
Total	$ 14,665	$ 20,164