Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 1,477,670	$ 1,465,034
Gross Unrealized Gains	124	136
Gross Unrealized Losses	(57)	(94)
Fair Value	1,477,737	1,465,076
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	718,562	569,299
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	0
Fair Value	718,563	569,299
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	718,562	569,299
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	0
Fair Value	718,563	569,299
Available-for-sale Securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	759,108	895,735
Gross Unrealized Gains	123	136
Gross Unrealized Losses	(57)	(94)
Fair Value	759,174	895,777
Government-sponsored enterprise securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	467,857	600,496
Gross Unrealized Gains	27	7
Gross Unrealized Losses	(16)	(53)
Fair Value	467,868	600,450
Commercial paper (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	46,402	83,384
Gross Unrealized Gains	94	109
Gross Unrealized Losses	0	0
Fair Value	46,496	83,493
Corporate debt securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	213,352	189,674
Gross Unrealized Gains	2	14
Gross Unrealized Losses	0	(34)
Fair Value	213,354	189,654
Corporate debt securities (due after 1 year through 5 years)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	31,497	22,181
Gross Unrealized Gains	0	6
Gross Unrealized Losses	(41)	(7)
Fair Value	$ 31,456	$ 22,180