Investments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Summary of Investments
Available-for-sale securities, Amortized Cost	$ 184,082	$ 186,701
Available-for-sale securities, Gross Unrealized Gains	360	237
Available-for-sale securities, Gross Unrealized Losses	(26)	(135)
Available-for-sale securities, Estimated Fair Value	184,416	186,803
Corporate Debt Securities [Member]
Summary of Investments
Available-for-sale securities due in one year or less, Amortized Cost	41,237	42,609
Available-for-sale securities due in one to three years, Amortized Cost	89,780	91,443
Available-for-sale securities due in one year or less, Gross Unrealized Gains	68	44
Available-for-sale securities due in one to three years, Gross Unrealized Gains	210	137
Available-for-sale securities due in one year or less, Gross Unrealized Losses	(4)	(4)
Available-for-sale securities due in one to three years, Gross Unrealized Losses	(17)	(106)
Available-for-sale securities due in one year or less, Estimated Fair Value	41,301	42,649
Available-for-sale securities due in one to three years, Estimated Fair Value	89,973	91,474
U S Treasury And Government [Member]
Summary of Investments
Available-for-sale securities due in one year or less, Amortized Cost	18,848	18,526
Available-for-sale securities due in one to three years, Amortized Cost	32,717	34,123
Available-for-sale securities due in one year or less, Gross Unrealized Gains	31	19
Available-for-sale securities due in one to three years, Gross Unrealized Gains	51	37
Available-for-sale securities due in one to three years, Gross Unrealized Losses	(5)	(25)
Available-for-sale securities due in one year or less, Estimated Fair Value	18,879	18,545
Available-for-sale securities due in one to three years, Estimated Fair Value	32,763	34,135
Commercial Paper Not Included With Cash And Cash Equivalents [Member]
Summary of Investments
Available-for-sale securities due in one year or less, Amortized Cost	1,500
Available-for-sale securities due in one year or less, Estimated Fair Value	$ 1,500