Marketable Securities - Carrying Amounts of Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 35,010	$ 67,775	$ 47,814
Long-term marketable securities	46,093	65,015	69,991
Total marketable securities	81,103	132,790	117,805
U.S. Government and agency obligations [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	30,000	17,910	12,909
Long-term marketable securities	46,093	59,993	69,991
Commercial paper [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	0	49,865	34,905
Corporate bonds [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	5,010	0	0
Long-term marketable securities	$ 0	$ 5,022	$ 0