Marketable Securities - Carrying Amounts of Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 43,001	$ 25,043	$ 19,282
Long-term marketable securities	72,653	80,652	80,522
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	43,001	25,043	19,282
Long-term marketable securities	72,653	80,652	80,522
Total marketable securities	115,654	105,695	99,804
U.S. Government and agency obligations [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	23,024	15,051	9,290
Long-term marketable securities	72,653	80,652	80,522
Commercial paper [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 19,977	$ 9,992	$ 9,992