Cash Equivalents and Investments Classified as Available-for-sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Schedule Of Available For Sale Securities [Line Items]
Cost	$ 215,082	$ 127,041
Unrealized Gains	74	36
Unrealized Losses	(10)	(3)
Estimated Fair Value	215,146	127,074
Money market funds
Schedule Of Available For Sale Securities [Line Items]
Cost	54,929	43,303
Unrealized Gains
Unrealized Losses
Estimated Fair Value	54,929	43,303
Corporate notes and commercial paper
Schedule Of Available For Sale Securities [Line Items]
Cost	103,034	64,403
Unrealized Gains	40	25
Unrealized Losses	(7)	(3)
Estimated Fair Value	103,067	64,425
U.S. government agency securities
Schedule Of Available For Sale Securities [Line Items]
Cost	57,119	19,335
Unrealized Gains	34	11
Unrealized Losses	(3)
Estimated Fair Value	57,150	19,346
Cash equivalents
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	54,929	43,303
Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	116,066	77,656
Long-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	$ 44,151	$ 6,115