Cash, Cash Equivalents and Marketable Securities Available-for-Sale (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Cash and cash equivalents
Money market funds	$ 667,300,000	$ 738,700,000
Available-for-sale securities by major security type and class of security
Amortized Cost	698,618,000	789,523,000
Gross Unrealized Gain	1,218,000	1,408,000
Gross Unrealized Loss	(771,000)	(2,241,000)
Estimated Fair Value	699,065,000	788,690,000
U.S. Treasury securities
Available-for-sale securities by major security type and class of security
Amortized Cost	418,287,000	228,996,000
Gross Unrealized Gain	20,000	58,000
Gross Unrealized Loss	(90,000)	(38,000)
Estimated Fair Value	418,217,000	229,016,000
U.S. government-sponsored agency securities
Available-for-sale securities by major security type and class of security
Amortized Cost	67,377,000	196,833,000
Gross Unrealized Gain	49,000	81,000
Gross Unrealized Loss	(4,000)	(69,000)
Estimated Fair Value	67,422,000	196,845,000
U.S. government-sponsored agency MBS
Available-for-sale securities by major security type and class of security
Amortized Cost	130,425,000	256,440,000
Gross Unrealized Gain	591,000	600,000
Gross Unrealized Loss	(636,000)	(1,901,000)
Estimated Fair Value	130,380,000	255,139,000
Non-U.S. government, agency and Supranational securities
Available-for-sale securities by major security type and class of security
Amortized Cost	2,668,000	2,666,000
Gross Unrealized Gain	24,000	19,000
Estimated Fair Value	2,692,000	2,685,000
Corporate debt - global
Available-for-sale securities by major security type and class of security
Amortized Cost	79,454,000	104,181,000
Gross Unrealized Gain	509,000	497,000
Gross Unrealized Loss	(41,000)	(233,000)
Estimated Fair Value	79,922,000	104,445,000
Marketable equity securities
Available-for-sale securities by major security type and class of security
Amortized Cost	407,000	407,000
Gross Unrealized Gain	25,000	153,000
Estimated Fair Value	$ 432,000	$ 560,000