Basis of Presentation and Summary of Significant Accounting Policies (Details) (Recurring basis, USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair value of assets 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 measured on a recurring basis
Fair value of assets	204,766	204,808
Total | Money market funds
Fair value of assets measured on a recurring basis
Fair value of assets	21,542	24,058
Total | Corporate debt securities
Fair value of assets measured on a recurring basis
Fair value of assets	124,710	111,690
Total | U.S. treasury and government agency securities
Fair value of assets measured on a recurring basis
Fair value of assets	54,517	59,569
Total | Commercial paper
Fair value of assets measured on a recurring basis
Fair value of assets	3,997	9,491
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value of assets measured on a recurring basis
Fair value of assets	21,542	24,058
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds
Fair value of assets measured on a recurring basis
Fair value of assets	21,542	24,058
Significant Other Observable Inputs (Level 2)
Fair value of assets measured on a recurring basis
Fair value of assets	183,224	180,750
Significant Other Observable Inputs (Level 2) | Corporate debt securities
Fair value of assets measured on a recurring basis
Fair value of assets	124,710	111,690
Significant Other Observable Inputs (Level 2) | U.S. treasury and government agency securities
Fair value of assets measured on a recurring basis
Fair value of assets	54,517	59,569
Significant Other Observable Inputs (Level 2) | Commercial paper
Fair value of assets measured on a recurring basis
Fair value of assets	$ 3,997	$ 9,491