Marketable Securities - Carrying Amounts of Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 67,824	$ 64,884	$ 43,001
Long-term marketable securities	59,989	62,895	72,653
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	67,824	64,884	43,001
Long-term marketable securities	59,989	62,895	72,653
Total marketable securities	127,813	127,779	115,654
U.S. Government and agency obligations [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	12,908	10,002	23,024
Long-term marketable securities	59,989	62,895	72,653
Commercial paper | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 54,916	$ 54,882	$ 19,977