Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 980,826	$ 969,031
Gross Unrealized Gains	149	10
Gross Unrealized Losses	(108)	(119)
Fair Value	980,867	968,922
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	267,923	475,337
Gross Unrealized Losses		(17)
Fair Value	267,923	475,320
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	261,523	362,035
Fair Value	261,523	362,035
Government-sponsored enterprise securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	2,150	113,302
Gross Unrealized Losses		(17)
Fair Value	2,150	113,285
Commercial paper
Summary of cash, cash equivalents and marketable securities
Amortized Cost	4,250
Fair Value	4,250
Total marketable securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	712,903	493,694
Gross Unrealized Gains	149	10
Gross Unrealized Losses	(108)	(102)
Fair Value	712,944	493,602
U.S. Treasury securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	26,200	22,105
Gross Unrealized Gains		2
Gross Unrealized Losses	(3)
Fair Value	26,197	22,107
Government-sponsored enterprise securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	495,375	471,589
Gross Unrealized Gains	2	8
Gross Unrealized Losses	(59)	(102)
Fair Value	495,318	471,495
Commercial paper (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	145,305
Gross Unrealized Gains	142
Fair Value	145,447
Corporate debt securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	46,023
Gross Unrealized Gains	5
Gross Unrealized Losses	(46)
Fair Value	$ 45,982