Marketable Securities (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 658,717	$ 1,031,401
Gross Unrealized Gains	7	89
Gross Unrealized Losses	(39)	(79)
Fair Value	658,685	1,031,411
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	542,573	243,202
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(29)	(6)
Fair Value	542,545	243,197
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	348,199	193,845
Fair Value	348,199	193,845
U.S. Treasury securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	7,788	4,770
Fair Value	7,788	4,770
Government-sponsored enterprise securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	186,586	44,587
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(29)	(6)
Fair Value	186,558	44,582
Total marketable securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	116,144	788,199
Gross Unrealized Gains	6	88
Gross Unrealized Losses	(10)	(73)
Fair Value	116,140	788,214
U.S. Treasury securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost		103,230
Gross Unrealized Gains		1
Gross Unrealized Losses		(11)
Fair Value		103,220
Government-sponsored enterprise securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	116,144	684,969
Gross Unrealized Gains	6	87
Gross Unrealized Losses	(10)	(62)
Fair Value	$ 116,140	$ 684,994