Cash Equivalents and Investments Classified as Available-for-sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]
Cost	$ 264,896	$ 315,000
Unrealized Gain	69	85
Unrealized (Loss)	(47)	(28)
Estimated Fair Value	264,918	315,057
Money market funds
Schedule Of Available For Sale Securities [Line Items]
Cost	6,692	57,296
Estimated Fair Value	6,692	57,296
Corporate notes and commercial paper
Schedule Of Available For Sale Securities [Line Items]
Cost	191,153	182,426
Unrealized Gain	48	62
Unrealized (Loss)	(43)	(16)
Estimated Fair Value	191,158	182,472
U.S. government agency securities
Schedule Of Available For Sale Securities [Line Items]
Cost	67,051	75,278
Unrealized Gain	21	23
Unrealized (Loss)	(4)	(12)
Estimated Fair Value	67,068	75,289
Cash equivalents
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	36,059	113,794
Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	170,857	150,892
Long-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	$ 58,002	$ 50,371