Marketable Securities (Tables)

3 Months Ended

Mar. 31, 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 March 31, 2012
Cash and cash equivalents:
Cash and money market funds	$	261,523	$	—	$	—		$	261,523
Government-sponsored enterprise securities		2,150		—		—			2,150
Commercial paper		4,250		—		—			4,250
Total cash and cash equivalents	$	267,923	$	—	$	—		$	267,923
Marketable securities:
U.S. Treasury securities (due within 1 year)	$	26,200	$	—	$	(3	)	$	26,197
Government-sponsored enterprise securities (due within 1 year)		495,375		2		(59	)		495,318
Commercial paper (due within 1 year)		145,305		142		—			145,447
Corporate debt securities (due within 1 year)		46,023		5		(46	)		45,982
Total marketable securities	$	712,903	$	149	$	(108	)	$	712,944
Total cash, cash equivalents and marketable securities	$	980,826	$	149	$	(108	)	$	980,867
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