Marketable Securities (Tables)

6 Months Ended

Jun. 30, 2012

Marketable Securities

Summary of cash, cash equivalents and marketable securities

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
As of June 30, 2012
Cash and cash equivalents:
Cash and money market funds	$	272,840	$	—	$	—		$	272,840
U.S. Treasury securities		181,234		—		(13	)		181,221
Total cash and cash equivalents	$	454,074	$	—	$	(13	)	$	454,061
Marketable securities:
U.S. Treasury securities (due within 1 year)	$	114,060	$	2	$	(7	)	$	114,055
Government-sponsored enterprise securities (due within 1 year)		371,170		15		(27	)		371,158
Commercial paper (due within 1 year)		229,243		192		—			229,435
Corporate debt securities (due within 1 year)		54,641		—		(16	)		54,625
Total marketable securities	$	769,114	$	209	$	(50	)	$	769,273
Total cash, cash equivalents and marketable securities	$	1,223,188	$	209	$	(63	)	$	1,223,334
As of December 31, 2011
Cash and cash equivalents:
Cash and money market funds	$	362,035	$	—	$	—		$	362,035
Government-sponsored enterprise securities		113,302		—		(17	)		113,285
Total cash and cash equivalents	$	475,337	$	—	$	(17	)	$	475,320
Marketable securities:
U.S. Treasury securities (due within 1 year)	$	22,105	$	2	$	—		$	22,107
Government-sponsored enterprise securities (due within 1 year)		471,589		8		(102	)		471,495
Total marketable securities	$	493,694	$	10	$	(102	)	$	493,602
Total cash, cash equivalents and marketable securities	$	969,031	$	10	$	(119	)	$	968,922