Investments - Investment Summary (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 44,341	$ 54,063
Gross Unrealized Gains	197	225
Gross Unrealized Losses	35	261
Fair Value	44,503	54,027
U.S. Treasury bills
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	28,119	28,306
Gross Unrealized Gains	116	115
Gross Unrealized Losses	27	181
Fair Value	28,208	28,240
Corporate bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	16,222	25,757
Gross Unrealized Gains	81	110
Gross Unrealized Losses	8	80
Fair Value	$ 16,295	$ 25,787