Fair Value Measurements (Details) (Recurring basis, USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Quoted Prices in Active markets for Identical Assets, Level 1
Estimated fair values of entity's financial assets
Fair value of financial assets	$ 163,501	$ 176,851
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	66,174	25,976
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	55,901	24,375
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	34,695
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	38,721	91,805
Significant Other Observable Inputs, Level 2
Estimated fair values of entity's financial assets
Fair value of financial assets	72,250	127,494
Significant Other Observable Inputs, Level 2 | U.S. government agency securities
Estimated fair values of entity's financial assets
Fair value of financial assets	37,274	30,273
Significant Other Observable Inputs, Level 2 | U.S. commercial paper
Estimated fair values of entity's financial assets
Fair value of financial assets	34,976	97,221
Total
Estimated fair values of entity's financial assets
Fair value of financial assets	235,751	304,345
Total | U.S. government securities
Estimated fair values of entity's financial assets
Fair value of financial assets	66,174	25,976
Total | U.S. government agency securities
Estimated fair values of entity's financial assets
Fair value of financial assets	93,175	54,648
Total | U.S. corporate notes
Estimated fair values of entity's financial assets
Fair value of financial assets	2,705	34,695
Total | U.S. commercial paper
Estimated fair values of entity's financial assets
Fair value of financial assets	34,976	97,221
Total | Money market funds
Estimated fair values of entity's financial assets
Fair value of financial assets	$ 38,721	$ 91,805