Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 1,219,064	$ 1,465,034
Gross Unrealized Gains	145	136
Gross Unrealized Losses	(48)	(94)
Fair Value	1,219,161	1,465,076
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	420,558	569,299
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	420,558	569,299
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	420,558	569,299
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	420,558	569,299
Available-for-sale Securities [Member]
Summary of cash, cash equivalents and marketable securities
Amortized Cost	798,506	895,735
Gross Unrealized Gains	145	136
Gross Unrealized Losses	(48)	(94)
Fair Value	798,603	895,777
Government-sponsored enterprise securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	497,601	600,496
Gross Unrealized Gains	8	7
Gross Unrealized Losses	(22)	(53)
Fair Value	497,587	600,450
Commercial paper (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	69,144	83,384
Gross Unrealized Gains	99	109
Gross Unrealized Losses	0	0
Fair Value	69,243	83,493
Corporate debt securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	216,426	189,674
Gross Unrealized Gains	33	14
Gross Unrealized Losses	(26)	(34)
Fair Value	216,433	189,654
Corporate debt securities (due after 1 year through 5 years)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	15,335	22,181
Gross Unrealized Gains	5	6
Gross Unrealized Losses	0	(7)
Fair Value	$ 15,340	$ 22,180