Fair Value Measurements (Details) (Assets and liabilities measured on a recurring basis, USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Level 1
Major categories of assets and liabilities measured on a recurring basis, at fair value
Total	$ 271,201	$ 278,698
Level 1 | Treasury bills
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	271,201	278,698
Level 2
Major categories of assets and liabilities measured on a recurring basis, at fair value
Total	131,545	123,054
Level 2 | Government agency securities
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	48,054	123,054
Level 2 | Corporate bonds
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	63,499
Level 2 | Commercial paper
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	19,992
Fair Value
Major categories of assets and liabilities measured on a recurring basis, at fair value
Total	402,746	401,752
Fair Value | Treasury bills
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	271,201	278,698
Fair Value | Government agency securities
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	48,054	123,054
Fair Value | Corporate bonds
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	63,499
Fair Value | Commercial paper
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	$ 19,992