Investments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Summary of Investments
Available-for-sale securities, Amortized Cost	$ 186,701	$ 180,403
Available-for-sale securities, Gross Unrealized Gains	237	396
Available-for-sale securities, Gross Unrealized Losses	(135)	(49)
Available-for-sale securities, Estimated Fair Value	186,803	180,750
Corporate debt securities
Summary of Investments
Available-for-sale securities due in one year or less, Amortized Cost	42,609	52,332
Available-for-sale securities due in one to three years, Amortized Cost	91,443	59,176
Available-for-sale securities due in one year or less, Gross Unrealized Gains	44	88
Available-for-sale securities due in one to three years, Gross Unrealized Gains	137	137
Available-for-sale securities due in one year or less, Gross Unrealized Losses	(4)	(6)
Available-for-sale securities due in one to three years, Gross Unrealized Losses	(106)	(37)
Available-for-sale securities due in one year or less, Estimated Fair Value	42,649	52,414
Available-for-sale securities due in one to three years, Estimated Fair Value	91,474	59,276
U.S. treasury and government agency securities
Summary of Investments
Available-for-sale securities due in one year or less, Amortized Cost	18,526	24,795
Available-for-sale securities due in one to three years, Amortized Cost	34,123	34,606
Available-for-sale securities due in one year or less, Gross Unrealized Gains	19	86
Available-for-sale securities due in one to three years, Gross Unrealized Gains	37	84
Available-for-sale securities due in one to three years, Gross Unrealized Losses	(25)	(2)
Available-for-sale securities due in one year or less, Estimated Fair Value	18,545	24,881
Available-for-sale securities due in one to three years, Estimated Fair Value	34,135	34,688
Commercial paper
Summary of Investments
Available-for-sale securities due in one year or less, Amortized Cost		9,494
Available-for-sale securities due in one year or less, Gross Unrealized Gains		1
Available-for-sale securities due in one year or less, Gross Unrealized Losses		(4)
Available-for-sale securities due in one year or less, Estimated Fair Value		$ 9,491