Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 1,465,034	$ 1,321,019
Gross Unrealized Gains	136	217
Gross Unrealized Losses	(94)	(21)
Fair Value	1,465,076	1,321,215
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	569,299	489,407
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	569,299	489,407
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	569,299	489,407
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	569,299	489,407
Total marketable securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	895,735	831,612
Gross Unrealized Gains	136	217
Gross Unrealized Losses	(94)	(21)
Fair Value	895,777	831,808
U.S. Treasury securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost		111,350
Gross Unrealized Gains		2
Gross Unrealized Losses		(2)
Fair Value		111,350
Government-sponsored enterprise securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	600,496	440,181
Gross Unrealized Gains	7	49
Gross Unrealized Losses	(53)	(5)
Fair Value	600,450	440,225
Commercial paper (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	83,384	225,294
Gross Unrealized Gains	109	155
Gross Unrealized Losses	0	0
Fair Value	83,493	225,449
Corporate debt securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	189,674	15,429
Gross Unrealized Gains	14	1
Gross Unrealized Losses	(34)	(1)
Fair Value	189,654	15,429
Corporate debt securities (due after 1 year through 5 years)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	22,181	39,358
Gross Unrealized Gains	6	10
Gross Unrealized Losses	(7)	(13)
Fair Value	$ 22,180	$ 39,355