Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 1,422,447	$ 1,321,019
Gross Unrealized Gains	252	217
Gross Unrealized Losses	(49)	(21)
Fair Value	1,422,650	1,321,215
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	583,181	489,407
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	583,181	489,407
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	580,286	489,407
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	580,286	489,407
Government-sponsored enterprise securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	2,895	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	2,895	0
Available-for-sale Securities [Member]
Summary of cash, cash equivalents and marketable securities
Gross Unrealized Gains	252	217
Gross Unrealized Losses	(49)	(21)
Available-for-sale Securities, Amortized Cost Basis	839,266	831,612
Available-for-sale Securities, Fair Value Disclosure	839,469	831,808
U.S. Treasury securities
Summary of cash, cash equivalents and marketable securities
Gross Unrealized Gains	0	2
Gross Unrealized Losses	0	(2)
Available-for-sale Securities, Amortized Cost Basis	0	111,350
Available-for-sale Securities, Fair Value Disclosure	0	111,350
Government-sponsored enterprise securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Gross Unrealized Gains	35	49
Gross Unrealized Losses	(10)	(5)
Available-for-sale Securities, Amortized Cost Basis	555,502	440,181
Available-for-sale Securities, Fair Value Disclosure	555,527	440,225
Commercial paper
Summary of cash, cash equivalents and marketable securities
Gross Unrealized Gains	191	155
Gross Unrealized Losses	0	0
Available-for-sale Securities, Amortized Cost Basis	125,073	225,294
Available-for-sale Securities, Fair Value Disclosure	125,264	225,449
Corporate debt securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Gross Unrealized Gains	21	1
Gross Unrealized Losses	(38)	(1)
Available-for-sale Securities, Amortized Cost Basis	148,672	15,429
Available-for-sale Securities, Fair Value Disclosure	148,655	15,429
Corporate debt securities (due after 1 year through 5 years)
Summary of cash, cash equivalents and marketable securities
Gross Unrealized Gains	5	10
Gross Unrealized Losses	(1)	(13)
Available-for-sale Securities, Amortized Cost Basis	10,019	39,358
Available-for-sale Securities, Fair Value Disclosure	$ 10,023	$ 39,355