Marketable Securities (Details) (Prepaid Expenses and Other Current Assets [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Short-term marketable securities
Cost	$ 350,724		$ 297,389
Unrealized Gain	43		107
Unrealized Loss	(46)		(44)
Fair value	350,721		297,452
U.S. treasury and agency securities
Short-term marketable securities
Cost	97,085		85,537
Unrealized Gain	14	[1]	32	[1]
Unrealized Loss	(2)	[1]	(1)	[1]
Fair value	97,097		85,568
Corporate debt securities
Short-term marketable securities
Cost	253,639		211,852
Unrealized Gain	29	[1]	75	[1]
Unrealized Loss	(44)	[1]	(43)	[1]
Fair value	$ 253,624		$ 211,884

[1] Unrealized gains and losses are recorded as a component of stockholders’ equity.