Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 1,324,185	$ 1,465,034
Gross Unrealized Gains	95	136
Gross Unrealized Losses	(80)	(94)
Fair Value	1,324,200	1,465,076
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	424,045	569,299
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	424,045	569,299
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	424,045	569,299
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	424,045	569,299
Available-for-sale Securities [Member]
Summary of cash, cash equivalents and marketable securities
Amortized Cost	900,140	895,735
Gross Unrealized Gains	95	136
Gross Unrealized Losses	(80)	(94)
Fair Value	900,155	895,777
Government-sponsored enterprise securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	607,934	600,496
Gross Unrealized Gains	16	7
Gross Unrealized Losses	(28)	(53)
Fair Value	607,922	600,450
Commercial paper (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	80,950	83,384
Gross Unrealized Gains	60	109
Gross Unrealized Losses	0	0
Fair Value	81,010	83,493
Corporate debt securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	196,420	189,674
Gross Unrealized Gains	18	14
Gross Unrealized Losses	(45)	(34)
Fair Value	196,393	189,654
Corporate debt securities (due after 1 year through 5 years)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	14,836	22,181
Gross Unrealized Gains	1	6
Gross Unrealized Losses	(7)	(7)
Fair Value	$ 14,830	$ 22,180