Marketable Securities and Certificates of Deposit - Scheduled Maturities of Available-for-Sale Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Due within one year, amortized cost	$ 200	$ 204
Due after one year through five years, amortized cost	999	1,602
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Amortized Cost Basis	99	0
Total, amortized cost	1,298	1,806
Due within one year, fair value	200	204
Due after one year through five years, fair value	995	1,591
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value	98	0
Total, fair value	$ 1,293	$ 1,795