Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 1,321,019	$ 969,031
Gross Unrealized Gains	217	10
Gross Unrealized Losses	(21)	(119)
Fair Value	1,321,215	968,922
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	489,407	475,337
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(17)
Fair Value	489,407	475,320
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	489,407	362,035
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	489,407	362,035
Government-sponsored enterprise securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost		113,302
Gross Unrealized Gains		0
Gross Unrealized Losses		(17)
Fair Value		113,285
Total marketable securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	831,612	493,694
Gross Unrealized Gains	217	10
Gross Unrealized Losses	(21)	(102)
Fair Value	831,808	493,602
U.S. Treasury securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	111,350	22,105
Gross Unrealized Gains	2	2
Gross Unrealized Losses	(2)	0
Fair Value	111,350	22,107
Government-sponsored enterprise securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	440,181	471,589
Gross Unrealized Gains	49	8
Gross Unrealized Losses	(5)	(102)
Fair Value	440,225	471,495
Commercial paper (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	225,294
Gross Unrealized Gains	155
Gross Unrealized Losses	0
Fair Value	225,449
Corporate debt securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	15,429
Gross Unrealized Gains	1
Gross Unrealized Losses	(1)
Fair Value	15,429
Corporate debt securities (due after 1 year through 5 years)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	39,358
Gross Unrealized Gains	10
Gross Unrealized Losses	(13)
Fair Value	$ 39,355