Fair Value Measurements - Fair Value of Financial Assets and Liabilities, Allocated into Level 1, Level 2, and Level 3 Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value Measurement Inputs Disclosure [Line Items]
Amortized Cost	$ 415,405	$ 296,133
Unrealized Gain	12	2
Unrealized (Loss)	(15)	(286)
Estimated Fair Value	415,402	295,849
Money market funds \| Level 1
Fair Value Measurement Inputs Disclosure [Line Items]
Amortized Cost	88,619	19,500
Unrealized Gain	0	0
Unrealized (Loss)	0	0
Estimated Fair Value	88,619	19,500
Corporate notes and commercial paper \| Level 2
Fair Value Measurement Inputs Disclosure [Line Items]
Amortized Cost	228,354	166,363
Unrealized Gain	2	1
Unrealized (Loss)	(8)	(205)
Estimated Fair Value	228,348	166,159
U.S. Treasury bills and government agency securities \| Level 2
Fair Value Measurement Inputs Disclosure [Line Items]
Amortized Cost	98,432	110,270
Unrealized Gain	10	1
Unrealized (Loss)	(7)	(81)
Estimated Fair Value	98,435	110,190
Cash equivalents
Fair Value Measurement Inputs Disclosure [Line Items]
Estimated Fair Value	242,860	117,836
Short-term investments
Fair Value Measurement Inputs Disclosure [Line Items]
Estimated Fair Value	$ 172,542	$ 178,013

Fair value measurement inputs disclosure.

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
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-Section 50
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Publisher FASB
-Name Accounting Standards Codification
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