Basis of Presentation and Summary of Significant Accounting Policies (Details) (Recurring basis, USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Total
Fair value of assets measured on a recurring basis
Fair value of assets	$ 237,900	$ 291,454
Total | Money market funds
Fair value of assets measured on a recurring basis
Fair value of assets	62,709	110,238
Total | Corporate debt securities
Fair value of assets measured on a recurring basis
Fair value of assets	92,757	83,768
Total | U.S. treasury and government agency securities
Fair value of assets measured on a recurring basis
Fair value of assets	44,310	50,925
Total | Foreign government securities
Fair value of assets measured on a recurring basis
Fair value of assets		2,431
Total | Commercial paper
Fair value of assets measured on a recurring basis
Fair value of assets	12,993	10,495
Total | Auction rate securities
Fair value of assets measured on a recurring basis
Fair value of assets	25,131	33,597
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value of assets measured on a recurring basis
Fair value of assets	62,709	110,238
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	62,709	110,238
Significant Other Observable Inputs (Level 2)
Fair value of assets measured on a recurring basis
Fair value of assets	150,060	147,619
Significant Other Observable Inputs (Level 2) | Corporate debt securities
Fair value of assets measured on a recurring basis
Fair value of assets	92,757	83,768
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	44,310	50,925
Significant Other Observable Inputs (Level 2) | Foreign government securities
Fair value of assets measured on a recurring basis
Fair value of assets		2,431
Significant Other Observable Inputs (Level 2) | Commercial paper
Fair value of assets measured on a recurring basis
Fair value of assets	12,993	10,495
Significant Unobservable Inputs (Level 3)
Fair value of assets measured on a recurring basis
Fair value of assets	25,131	33,597
Significant Unobservable Inputs (Level 3) | Auction rate securities
Fair value of assets measured on a recurring basis
Fair value of assets	$ 25,131	$ 33,597