Fair Value Measurements (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Estimated fair values of entity's financial assets
Fair value of financial assets	$ 377,487,000	$ 235,751,000
Fair value measurements transfer to Level 1 From Level 2	11,200,000	11,400,000
U.S. government securities
Estimated fair values of entity's financial assets
Fair value of financial assets	30,255,000	66,174,000
U.S. government agency securities
Estimated fair values of entity's financial assets
Fair value of financial assets	112,196,000	93,175,000
U.S. corporate notes
Estimated fair values of entity's financial assets
Fair value of financial assets	60,152,000	2,705,000
U.S. commercial paper
Estimated fair values of entity's financial assets
Fair value of financial assets	44,993,000	34,976,000
Money market funds
Estimated fair values of entity's financial assets
Fair value of financial assets	129,891,000	38,721,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1
Estimated fair values of entity's financial assets
Fair value of financial assets	292,844,000	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
Fair value of financial assets	30,255,000	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
Fair value of financial assets	75,632,000	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
Fair value of financial assets	57,066,000	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
Fair value of financial assets	129,891,000	38,721,000
Recurring basis | Significant Other Observable Inputs, Level 2
Estimated fair values of entity's financial assets
Fair value of financial assets	84,643,000	72,250,000
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government agency securities
Estimated fair values of entity's financial assets
Fair value of financial assets	36,564,000	37,274,000
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. corporate notes
Estimated fair values of entity's financial assets
Fair value of financial assets	3,086,000
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. commercial paper
Estimated fair values of entity's financial assets
Fair value of financial assets	44,993,000	34,976,000
Recurring basis | Total
Estimated fair values of entity's financial assets
Fair value of financial assets	377,487,000	235,751,000
Recurring basis | Total | U.S. government securities
Estimated fair values of entity's financial assets
Fair value of financial assets	30,255,000	66,174,000
Recurring basis | Total | U.S. government agency securities
Estimated fair values of entity's financial assets
Fair value of financial assets	112,196,000	93,175,000
Recurring basis | Total | U.S. corporate notes
Estimated fair values of entity's financial assets
Fair value of financial assets	60,152,000	2,705,000
Recurring basis | Total | U.S. commercial paper
Estimated fair values of entity's financial assets
Fair value of financial assets	44,993,000	34,976,000
Recurring basis | Total | Money market funds
Estimated fair values of entity's financial assets
Fair value of financial assets	$ 129,891,000	$ 38,721,000