Marketable Securities - Carrying Amounts of Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 20,014	$ 67,775	$ 47,821
Long-term marketable securities	61,191	65,015	59,990
Total marketable securities	81,205	132,790	107,811
U.S. Government and agency obligations [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	15,000	17,910	12,909
Long-term marketable securities	61,191	59,993	59,990
Commercial paper [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	0	49,865	34,912
Corporate bonds [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	5,014	0	0
Long-term marketable securities	$ 0	$ 5,022	$ 0