Marketable Securities (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 593,479	$ 1,031,401
Gross Unrealized Gains	12	89
Gross Unrealized Losses		(79)
Fair Value	593,491	1,031,411
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	458,194	243,202
Gross Unrealized Gains	1	1
Gross Unrealized Losses		(6)
Fair Value	458,195	243,197
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	422,289	193,845
Fair Value	422,289	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	35,905	44,587
Gross Unrealized Gains	1	1
Gross Unrealized Losses		(6)
Fair Value	35,906	44,582
Total marketable securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	135,285	788,199
Gross Unrealized Gains	11	88
Gross Unrealized Losses		(73)
Fair Value	135,296	788,214
U.S. Treasury securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	9,546	103,230
Gross Unrealized Gains	1	1
Gross Unrealized Losses		(11)
Fair Value	9,547	103,220
Government-sponsored enterprise securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	125,739	684,969
Gross Unrealized Gains	10	87
Gross Unrealized Losses		(62)
Fair Value	$ 125,749	$ 684,994