Marketable Securities - Carrying Amounts of Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 47,814	$ 64,884	$ 54,863
Long-term marketable securities	69,991	62,895	52,908
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	47,814	64,884	54,863
Long-term marketable securities	69,991	62,895	52,908
Total marketable securities	117,805	127,779	107,771
U.S. Government and agency obligations [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	12,909	10,002	14,999
Long-term marketable securities	69,991	62,895	52,908
Commercial paper | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 34,905	$ 54,882	$ 39,864