Cash Equivalents and Investments Classified as Available-for-sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities [Line Items]
Cost	$ 343,349	$ 371,370
Unrealized Gain	53	12
Unrealized (Loss)	(34)	(251)
Estimated Fair Value	343,369	371,131
Money market funds
Schedule Of Available For Sale Securities [Line Items]
Cost	21,351	24,915
Estimated Fair Value	21,351	24,915
Corporate notes and commercial paper
Schedule Of Available For Sale Securities [Line Items]
Cost	181,981	226,209
Unrealized Gain	14	8
Unrealized (Loss)	(33)	(170)
Estimated Fair Value	181,962	226,047
U.S. government agency securities
Schedule Of Available For Sale Securities [Line Items]
Cost	140,017	120,246
Unrealized Gain	39	4
Unrealized (Loss)	(1)	(81)
Estimated Fair Value	140,056	120,169
Cash equivalents
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	65,123	36,342
Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	$ 278,246	251,759
Long-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value		$ 83,030

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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-Publisher FASB

-Name Accounting Standards Codification

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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