Marketable Securities - Carrying Amounts of Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 39,961	$ 67,775	$ 67,824
Long-term marketable securities	67,305	65,015	59,989
Total marketable securities	107,266	132,790	127,813
U.S. Government and agency obligations [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	15,000	17,910	12,908
Long-term marketable securities	62,287	59,993	59,989
Commercial paper
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	24,961	49,865	$ 54,916
Corporate bonds [Member]
Schedule of Marketable Securities [Line Items]
Long-term marketable securities	$ 5,018	$ 5,022