Cash Equivalents and Investments Classified as Available-for-sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]
Cost	$ 238,614	$ 315,000
Unrealized Gain	50	85
Unrealized (Loss)	(74)	(28)
Estimated Fair Value	238,590	315,057
Money market funds
Schedule Of Available For Sale Securities [Line Items]
Cost	13,588	57,296
Estimated Fair Value	13,588	57,296
Corporate notes and commercial paper
Schedule Of Available For Sale Securities [Line Items]
Cost	164,048	182,426
Unrealized Gain	30	62
Unrealized (Loss)	(72)	(16)
Estimated Fair Value	164,006	182,472
U.S. government agency securities
Schedule Of Available For Sale Securities [Line Items]
Cost	60,978	75,278
Unrealized Gain	20	23
Unrealized (Loss)	(2)	(12)
Estimated Fair Value	60,996	75,289
Cash equivalents
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	29,727	113,794
Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	172,060	150,892
Long-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	$ 36,803	$ 50,371