Cash Equivalents and Investments Classified as Available-for-sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Cost	$ 562,466	$ 195,300
Unrealized Gains	6	6
Unrealized (Losses)	(44)	(17)
Estimated Fair Value	562,428	195,289
Money market funds
Schedule Of Available For Sale Securities [Line Items]
Cost	107,888	6,254
Estimated Fair Value	107,888	6,254
Corporate notes and commercial paper
Schedule Of Available For Sale Securities [Line Items]
Cost	273,897	133,112
Unrealized Gains	1	1
Unrealized (Losses)	(10)	(14)
Estimated Fair Value	273,888	133,099
U.S. government agency securities
Schedule Of Available For Sale Securities [Line Items]
Cost	180,681	55,934
Unrealized Gains	5	5
Unrealized (Losses)	(34)	(3)
Estimated Fair Value	180,652	55,936
Cash equivalents
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	308,696	64,998
Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	225,107	$ 130,291
Long-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	$ 28,625

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
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-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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