Cash Equivalents and Investments Classified as Available-for-sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities [Line Items]
Cost	$ 442,580	$ 371,370
Unrealized Gain	39	12
Unrealized (Loss)	(110)	(251)
Estimated Fair Value	442,509us-gaap_AvailableForSaleSecurities	371,131us-gaap_AvailableForSaleSecurities
Money market funds
Schedule Of Available For Sale Securities [Line Items]
Cost	55,862	24,915
Estimated Fair Value	55,862	24,915
Corporate notes and commercial paper
Schedule Of Available For Sale Securities [Line Items]
Cost	255,007	226,209
Unrealized Gain	23	8
Unrealized (Loss)	(91)	(170)
Estimated Fair Value	254,939	226,047
U.S. government agency securities
Schedule Of Available For Sale Securities [Line Items]
Cost	131,711	120,246
Unrealized Gain	16	4
Unrealized (Loss)	(19)	(81)
Estimated Fair Value	131,708	120,169
Cash equivalents
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	118,982	36,342
Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	258,567	251,759
Long-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	$ 64,960	$ 83,030

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

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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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