Fair Value Measurements (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Estimated fair values of entity's financial assets
Fair value measurements transfer to Level 1 From Level 2	$ 19,600,000	$ 19,700,000
Fair value measured transfer to Level 2 From Level 1	56,200,000	56,500,000
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	66,652,000	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	25,998,000	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	34,441,000	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		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	6,213,000	38,721,000
Recurring basis | Significant Other Observable Inputs, Level 2
Estimated fair values of entity's financial assets
Fair value of financial assets	129,808,000	72,250,000
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government securities
Estimated fair values of entity's financial assets
Fair value of financial assets	35,033,000
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	48,488,000	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	550,000
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	45,737,000	34,976,000
Recurring basis | Total
Estimated fair values of entity's financial assets
Fair value of financial assets	196,460,000	235,751,000
Recurring basis | Total | U.S. government securities
Estimated fair values of entity's financial assets
Fair value of financial assets	61,031,000	66,174,000
Recurring basis | Total | U.S. government agency securities
Estimated fair values of entity's financial assets
Fair value of financial assets	82,929,000	93,175,000
Recurring basis | Total | U.S. corporate notes
Estimated fair values of entity's financial assets
Fair value of financial assets	550,000	2,705,000
Recurring basis | Total | U.S. commercial paper
Estimated fair values of entity's financial assets
Fair value of financial assets	45,737,000	34,976,000
Recurring basis | Total | Money market funds
Estimated fair values of entity's financial assets
Fair value of financial assets	$ 6,213,000	$ 38,721,000