Fair Value Measurements (Details) (Assets and liabilities measured on a recurring basis, USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Level 1
Major categories of assets and liabilities measured on a recurring basis, at fair value
Total	$ 206,061	$ 278,698
Level 1 | United States treasuries
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	206,061	278,698
Level 2
Major categories of assets and liabilities measured on a recurring basis, at fair value
Total	166,068	123,054
Level 2 | Commercial paper
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	4,000
Level 2 | Corporate bonds
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	75,628
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	86,440	123,054
Fair Value
Major categories of assets and liabilities measured on a recurring basis, at fair value
Total	372,129	401,752
Fair Value | United States treasuries
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	206,061	278,698
Fair Value | Commercial paper
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	4,000
Fair Value | Corporate bonds
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	75,628
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	$ 86,440	$ 123,054