Investments - Investment Summary (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Amortized Costs	$ 33,684	$ 44,341
Gross Unrealized Gains	64	197
Gross Unrealized Losses	30	35
Fair Value	33,718	44,503
U.S. Treasury bills
Schedule of Held-to-maturity Securities [Line Items]
Amortized Costs	28,073	28,119
Gross Unrealized Gains	40	116
Gross Unrealized Losses	29	27
Fair Value	28,084	28,208
Corporate bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Costs	5,611	16,222
Gross Unrealized Gains	24	81
Gross Unrealized Losses	1	8
Fair Value	$ 5,634	$ 16,295