Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2011

Marketable Securities

Schedule of estimated fair value of available-for-sale debt securities and money market funds

	December 31, 2011											December 31, 2010
(in thousands)	Amortized Cost			Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value			Amortized Cost			Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
U.S. government securities	$	66,150		$	24	$	—		$	66,174		$	25,966		$	10	$	—		$	25,976
U.S. government agencies		93,183			9		(17	)		93,175			54,625			30		(7	)		54,648
U.S. corporate notes		2,707			—		(2	)		2,705			34,695			9		(9	)		34,695
U.S. commercial paper		34,973			3		—			34,976			97,221			—		—			97,221
Money market funds		38,721			—		—			38,721			91,805			—		—			91,805
Total		235,734			36		(19	)		235,751			304,312			49		(16	)		304,345
Less amounts classified as cash equivalents		(38,721	)		—		—			(38,721	)		(157,151	)		—		—			(157,151	)
Less amounts classified as restricted cash		(893	)		—		—			(893	)		(893	)		—		—			(893	)
Amounts classified as marketable securities	$	196,120		$	36	$	(19	)	$	196,137		$	146,268		$	49	$	(16	)	$	146,301

Schedule of net realized gains (losses) on marketable securities

	Year Ended December 31,
(in thousands)	2011			2010		2009
Realized gains	$	—		$	3	$	—
Realized losses		(2	)		—		—
Net realized gains (losses)	$	(2	)	$	3	$	—

Summary of marketable securities with unrealized losses

	In loss position for less than 12 months					In loss position for more than 12 months				Total
(in thousands)	Fair Value		Estimated Gross Unrealized Losses			Fair Value		Estimated Gross Unrealized Losses		Fair Value		Estimated Gross Unrealized Losses
U.S. government agencies	$	47,807	$	(17	)	$	—	$	—	$	47,807	$	(17	)
U.S. corporate notes		2,754		(2	)		—		—		2,754		(2	)
Total	$	50,561	$	(19	)	$	—	$	—	$	50,561	$	(19	)