Marketable Securities - Carrying Amounts of Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 47,821	$ 64,884	$ 34,959
Long-term marketable securities	59,990	62,895	42,653
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	47,821	64,884	34,959
Long-term marketable securities	59,990	62,895	42,653
Total marketable securities	107,811	127,779	77,612
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	20,010
Long-term marketable securities	59,990	62,895	42,653
Commercial paper | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 34,912	$ 54,882	$ 14,949