Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 969,031	$ 1,031,401
Gross Unrealized Gains	10	89
Gross Unrealized Losses	(119)	(79)
Fair Value	968,922	1,031,411
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	475,337	243,202
Gross Unrealized Gains		1
Gross Unrealized Losses	(17)	(6)
Fair Value	475,320	243,197
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	362,035	193,845
Fair Value	362,035	193,845
U.S. Treasury securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost		4,770
Fair Value		4,770
Government-sponsored enterprise securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	113,302	44,587
Gross Unrealized Gains		1
Gross Unrealized Losses	(17)	(6)
Fair Value	113,285	44,582
Total marketable securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	493,694	788,199
Gross Unrealized Gains	10	88
Gross Unrealized Losses	(102)	(73)
Fair Value	493,602	788,214
U.S. Treasury securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	22,105	103,230
Gross Unrealized Gains	2	1
Gross Unrealized Losses		(11)
Fair Value	22,107	103,220
Government-sponsored enterprise securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	471,589	684,969
Gross Unrealized Gains	8	87
Gross Unrealized Losses	(102)	(62)
Fair Value	$ 471,495	$ 684,994