Cash Equivalents and Investments Classified as Available-for-sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 364,565	$ 515,867
Unrealized Gains	3	2
Unrealized (Losses)	(439)	(411)
Estimated Fair Value	364,129	515,458
Money market funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	15,062	31,836
Estimated Fair Value	15,062	31,836
Corporate notes and commercial paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	200,666	313,307
Unrealized Gains	3	2
Unrealized (Losses)	(238)	(145)
Estimated Fair Value	200,431	313,164
U.S. Treasury bills and government agency securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	148,837	170,724
Unrealized (Losses)	(201)	(266)
Estimated Fair Value	148,636	170,458
Cash equivalents
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	129,964	162,793
Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	223,284	281,589
Long-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	$ 10,881	$ 71,076

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

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

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

+ References

No definition available.

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- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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