Cash Equivalents and Investments Classified as Available-for-sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Schedule Of Available For Sale Securities [Line Items]
Cost	$ 315,000	$ 127,041
Unrealized Gain	85	36
Unrealized (Loss)	(28)	(3)
Estimated Fair Value	315,057	127,074
Money market funds
Schedule Of Available For Sale Securities [Line Items]
Cost	57,296	43,303
Unrealized Gain
Unrealized (Loss)
Estimated Fair Value	57,296	43,303
Corporate notes and commercial paper
Schedule Of Available For Sale Securities [Line Items]
Cost	182,426	64,403
Unrealized Gain	62	25
Unrealized (Loss)	(16)	(3)
Estimated Fair Value	182,472	64,425
U.S. government agency securities
Schedule Of Available For Sale Securities [Line Items]
Cost	75,278	19,335
Unrealized Gain	23	11
Unrealized (Loss)	(12)
Estimated Fair Value	75,289	19,346
Cash equivalents
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	113,794	43,303
Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	150,892	77,656
Long-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	$ 50,371	$ 6,115