Fair Value Measurements (Details) (Recurring basis, USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Fair value of assets and liabilities measured on a recurring basis
Transfers or reclassifications of securities from Level 1 to Level 2	$ 0
Transfers or reclassifications of securities from Level 2 to Level 1	0
Total
Fair value of assets and liabilities measured on a recurring basis
Total Assets	380,854	205,570
Total Liabilities	15,382	14,550
Total | Acquisition-related contingent consideration
Fair value of assets and liabilities measured on a recurring basis
Total Liabilities	15,382	14,550
Total | Money market funds
Fair value of assets and liabilities measured on a recurring basis
Total Assets	196,297	18,767
Total | Corporate debt securities
Fair value of assets and liabilities measured on a recurring basis
Total Assets	132,184	134,123
Total | U.S. treasury and government agency securities
Fair value of assets and liabilities measured on a recurring basis
Total Assets	50,875	52,680
Total | Commercial paper
Fair value of assets and liabilities measured on a recurring basis
Total Assets	1,498
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value of assets and liabilities measured on a recurring basis
Total Assets	196,297	18,767
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds
Fair value of assets and liabilities measured on a recurring basis
Total Assets	196,297	18,767
Significant Other Observable Inputs (Level 2)
Fair value of assets and liabilities measured on a recurring basis
Total Assets	184,557	186,803
Significant Other Observable Inputs (Level 2) | Corporate debt securities
Fair value of assets and liabilities measured on a recurring basis
Total Assets	132,184	134,123
Significant Other Observable Inputs (Level 2) | U.S. treasury and government agency securities
Fair value of assets and liabilities measured on a recurring basis
Total Assets	50,875	52,680
Significant Other Observable Inputs (Level 2) | Commercial paper
Fair value of assets and liabilities measured on a recurring basis
Total Assets	1,498
Significant Unobservable Inputs (Level 3)
Fair value of assets and liabilities measured on a recurring basis
Total Liabilities	15,382	14,550
Significant Unobservable Inputs (Level 3) | Acquisition-related contingent consideration
Fair value of assets and liabilities measured on a recurring basis
Total Liabilities	$ 15,382	$ 14,550