Marketable Securities - Carrying Amounts of Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 27,950	$ 49,968	$ 22,892
Long-term marketable securities	74,140	67,234	64,014
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	27,950	49,968	22,892
Long-term marketable securities	74,140	67,234	64,014
Total marketable securities	102,090	117,202	86,906
U.S. Government and agency obligations [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	12,967	10,000	1,260
Long-term marketable securities	74,140	67,234	64,014
Commercial paper [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	14,983	39,968	19,982
Municipal bonds [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 0	$ 0	$ 1,650