Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 1,181,081	$ 1,387,229
Gross Unrealized Gains	93	88
Gross Unrealized Losses	(40)	(211)
Fair Value	1,181,134	1,387,106
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	664,879	625,259
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	664,879	625,259
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	664,879	625,259
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	664,879	625,259
Total marketable securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	516,202	761,970
Gross Unrealized Gains	93	88
Gross Unrealized Losses	(40)	(211)
Fair Value	516,255	761,847
Government-sponsored enterprise securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	284,126	463,788
Gross Unrealized Gains	5	14
Gross Unrealized Losses	(16)	(52)
Fair Value	284,115	463,750
Commercial paper (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	59,688	51,674
Gross Unrealized Gains	57	72
Gross Unrealized Losses	0	0
Fair Value	59,745	51,746
Corporate debt securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	148,958	196,065
Gross Unrealized Gains	18	2
Gross Unrealized Losses	(24)	(66)
Fair Value	148,952	196,001
Corporate debt securities (due after 1 year through 5 years)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	23,430	50,443
Gross Unrealized Gains	13	0
Gross Unrealized Losses	0	(93)
Fair Value	$ 23,443	$ 50,350