Marketable Securities - Carrying Amounts of Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 54,863	$ 25,043	$ 17,607
Long-term marketable securities	52,908	80,652	70,646
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	54,863	25,043	17,607
Long-term marketable securities	52,908	80,652	70,646
Total marketable securities	107,771	105,695	88,253
U.S. Government and agency obligations [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	14,999	15,051	7,625
Long-term marketable securities	52,908	80,652	70,646
Commercial paper | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 39,864	$ 9,992	$ 9,982