Investments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Summary of Investments
Available-for-sale securities, Amortized Cost	$ 184,378	$ 186,701
Available-for-sale securities, Gross Unrealized Gains	257	237
Available-for-sale securities, Gross Unrealized Losses	(78)	(135)
Available-for-sale securities, Estimated Fair Value	184,557	186,803
Corporate debt securities
Summary of Investments
Available-for-sale securities due in one year or less, Amortized Cost	23,492	42,609
Available-for-sale securities due in one to three years, Amortized Cost	108,536	91,443
Available-for-sale securities due in one year or less, Gross Unrealized Gains	46	44
Available-for-sale securities due in one to three years, Gross Unrealized Gains	159	137
Available-for-sale securities due in one year or less, Gross Unrealized Losses	(2)	(4)
Available-for-sale securities due in one to three years, Gross Unrealized Losses	(47)	(106)
Available-for-sale securities due in one year or less, Estimated Fair Value	23,536	42,649
Available-for-sale securities due in one to three years, Estimated Fair Value	108,648	91,474
U.S. treasury and government agency securities
Summary of Investments
Available-for-sale securities due in one year or less, Amortized Cost	21,079	18,526
Available-for-sale securities due in one to three years, Amortized Cost	29,772	34,123
Available-for-sale securities due in one year or less, Gross Unrealized Gains	35	19
Available-for-sale securities due in one to three years, Gross Unrealized Gains	17	37
Available-for-sale securities due in one to three years, Gross Unrealized Losses	(28)	(25)
Available-for-sale securities due in one year or less, Estimated Fair Value	21,114	18,545
Available-for-sale securities due in one to three years, Estimated Fair Value	29,761	34,135
Commercial paper
Summary of Investments
Available-for-sale securities due in one year or less, Amortized Cost	1,499
Available-for-sale securities due in one year or less, Gross Unrealized Losses	(1)
Available-for-sale securities due in one year or less, Estimated Fair Value	$ 1,498