Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 M	Dec. 31, 2010
Available-for-sale securities
Less amounts classified as cash equivalents	$ (38,721)	$ (157,151)
Less amounts classified as restricted cash	(893)	(893)
Amortized Cost	196,120	146,268
Gross Unrealized Gains	36	49
Gross Unrealized Losses	(19)	(16)
Amounts classified as marketable securities	196,137	146,301
Net realized gains (losses) on marketable securities
Realized gains		3
Realized losses	(2)
Net realized gains (losses)	(2)	3
Fair Value
Fair Value, In loss position for less than 12 months	50,561
Fair Value, Total	50,561
Gross Unrealized Losses
Estimated Gross Unrealized Losses, In loss position for less than 12 months	(19)
Gross Unrealized Losses, Total	(19)
Maturity period for marketable securities
Maximum contractual maturity period (in months)	12 months
Average contractual maturity period (in months)	5
Available-for-sale securities and restricted and unrestricted cash
Available-for-sale securities
Amortized Cost	235,734	304,312
Gross Unrealized Gains	36	49
Gross Unrealized Losses	(19)	(16)
Estimated Fair Value	235,751	304,345
U.S. government securities | Available-for-sale securities and restricted and unrestricted cash
Available-for-sale securities
Amortized Cost	66,150	25,966
Gross Unrealized Gains	24	10
Estimated Fair Value	66,174	25,976
U.S. government agencies
Fair Value
Fair Value, In loss position for less than 12 months	47,807
Fair Value, Total	47,807
Gross Unrealized Losses
Estimated Gross Unrealized Losses, In loss position for less than 12 months	(17)
Gross Unrealized Losses, Total	(17)
U.S. government agencies | Available-for-sale securities and restricted and unrestricted cash
Available-for-sale securities
Amortized Cost	93,183	54,625
Gross Unrealized Gains	9	30
Gross Unrealized Losses	(17)	(7)
Estimated Fair Value	93,175	54,648
U.S. corporate notes
Fair Value
Fair Value, In loss position for less than 12 months	2,754
Fair Value, Total	2,754
Gross Unrealized Losses
Estimated Gross Unrealized Losses, In loss position for less than 12 months	(2)
Gross Unrealized Losses, Total	(2)
U.S. corporate notes | Available-for-sale securities and restricted and unrestricted cash
Available-for-sale securities
Amortized Cost	2,707	34,695
Gross Unrealized Gains		9
Gross Unrealized Losses	(2)	(9)
Estimated Fair Value	2,705	34,695
U.S. commercial paper | Available-for-sale securities and restricted and unrestricted cash
Available-for-sale securities
Amortized Cost	34,973	97,221
Gross Unrealized Gains	3
Estimated Fair Value	34,976	97,221
Cash and money market funds | Available-for-sale securities and restricted and unrestricted cash
Available-for-sale securities
Amortized Cost	38,721	91,805
Estimated Fair Value	$ 38,721	$ 91,805