Marketable Securities - Carrying Amounts of Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 41,143	$ 49,968	$ 44,841
Long-term marketable securities	65,969	67,234	57,958
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	41,143	49,968	44,841
Long-term marketable securities	65,969	67,234	57,958
Total marketable securities	107,112	117,202	102,799
U.S. Government and agency obligations [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	6,157	10,000	6,169
Long-term marketable securities	65,969	67,234	57,958
Commercial paper [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	34,986	39,968	29,976
Municipal bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 0	$ 0	$ 8,696