Securities Available for Sale - Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair value
Within 1 year	$ 59,914
After 1 year through 5 years	2,500
Total	128,424	$ 117,573
Amortized cost
Within 1 year	59,929
After 1 year through 5 years	2,500
Total	128,374
Debt securities, excluding asset-backed senior securities related to Company-sponsored securitizations
Fair value
Within 1 year	59,914
After 1 year through 5 years	2,500
Total	62,414
Certificates of deposit
Fair value
Within 1 year	20,303
After 1 year through 5 years	0
Total	20,303	24,758
Corporate debt securities
Fair value
Within 1 year	19,840
After 1 year through 5 years	0
Total	19,840	16,258
Asset-backed securities
Fair value
Within 1 year	12,086
After 1 year through 5 years	2,500
Total	14,586	14,843
Commercial paper
Fair value
Within 1 year	7,685
After 1 year through 5 years	0
Total	7,685	$ 14,665
Asset-backed senior securities related to Company-sponsored securitizations (1)
Fair value
Total	$ 66,010

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

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