Marketable Securities - Carrying Amounts of Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 19,282	$ 25,504	$ 41,143
Long-term marketable securities	80,522	76,563	65,969
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	19,282	25,504	41,143
Long-term marketable securities	80,522	76,563	65,969
Total marketable securities	99,804	102,067	107,112
U.S. Government and agency obligations [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	9,290	10,511	6,157
Long-term marketable securities	80,522	76,563	65,969
Commercial paper [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 9,992	$ 14,993	$ 34,986