Fair Value Measurements (Details) (USD $)	Sep. 30, 2012	Dec. 31, 2011
Estimated fair values of entity's financial assets
Total assets measured at fair value	$ 358,991,000	$ 235,751,000
Fair value measurements transfer to Level 1 From Level 2	11,100,000	11,400,000
Fair value measured transfer to Level 2 From Level 1	5,000,000	5,000,000
Fair value liabilities measurements transfer to Level 1 From Level 2	206,400,000	189,600,000
U.S. government securities
Estimated fair values of entity's financial assets
Total assets measured at fair value	30,242,000	66,174,000
U.S. government agency securities
Estimated fair values of entity's financial assets
Total assets measured at fair value	107,141,000	93,175,000
U.S. corporate notes
Estimated fair values of entity's financial assets
Total assets measured at fair value	98,637,000	2,705,000
U.S. commercial paper
Estimated fair values of entity's financial assets
Total assets measured at fair value	37,337,000	34,976,000
Money market funds
Estimated fair values of entity's financial assets
Total assets measured at fair value	85,634,000	38,721,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1
Estimated fair values of entity's financial assets
Total assets measured at fair value	255,929,000	163,501,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S. government securities
Estimated fair values of entity's financial assets
Total assets measured at fair value	30,242,000	66,174,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S. government agency securities
Estimated fair values of entity's financial assets
Total assets measured at fair value	58,893,000	55,901,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S. corporate notes
Estimated fair values of entity's financial assets
Total assets measured at fair value	81,160,000	2,705,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | Money market funds
Estimated fair values of entity's financial assets
Total assets measured at fair value	85,634,000	38,721,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | Convertible Subordinated Notes
Estimated fair values of entity's financial assets
Total liabilities measured at fair value	206,388,000
Recurring basis | Significant Other Observable Inputs, Level 2
Estimated fair values of entity's financial assets
Total assets measured at fair value	103,062,000	72,250,000
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government agency securities
Estimated fair values of entity's financial assets
Total assets measured at fair value	48,248,000	37,274,000
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. corporate notes
Estimated fair values of entity's financial assets
Total assets measured at fair value	17,477,000
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. commercial paper
Estimated fair values of entity's financial assets
Total assets measured at fair value	37,337,000	34,976,000
Recurring basis | Significant Other Observable Inputs, Level 2 | Convertible Subordinated Notes
Estimated fair values of entity's financial assets
Total liabilities measured at fair value		189,588,000
Recurring basis | Total
Estimated fair values of entity's financial assets
Total assets measured at fair value	358,991,000	235,751,000
Recurring basis | Total | U.S. government securities
Estimated fair values of entity's financial assets
Total assets measured at fair value	30,242,000	66,174,000
Recurring basis | Total | U.S. government agency securities
Estimated fair values of entity's financial assets
Total assets measured at fair value	107,141,000	93,175,000
Recurring basis | Total | U.S. corporate notes
Estimated fair values of entity's financial assets
Total assets measured at fair value	98,637,000	2,705,000
Recurring basis | Total | U.S. commercial paper
Estimated fair values of entity's financial assets
Total assets measured at fair value	37,337,000	34,976,000
Recurring basis | Total | Money market funds
Estimated fair values of entity's financial assets
Total assets measured at fair value	85,634,000	38,721,000
Recurring basis | Total | Convertible Subordinated Notes
Estimated fair values of entity's financial assets
Total liabilities measured at fair value	$ 206,388,000	$ 189,588,000