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	Jun. 30, 2019	Dec. 31, 2018
Fair Value Measurement Inputs Disclosure [Line Items]
Amortized Cost	$ 229,453	$ 296,133
Unrealized Gains	28	2
Unrealized (Losses)	(25)	(286)
Estimated Fair Value	229,456	295,849
Money market funds \| Level 1
Fair Value Measurement Inputs Disclosure [Line Items]
Amortized Cost	36,826	19,500
Estimated Fair Value	36,826	19,500
Cash equivalents
Fair Value Measurement Inputs Disclosure [Line Items]
Estimated Fair Value	140,903	117,836
Corporate notes and commercial paper \| Level 2
Fair Value Measurement Inputs Disclosure [Line Items]
Amortized Cost	127,459	166,363
Unrealized Gains	3	1
Unrealized (Losses)	(25)	(205)
Estimated Fair Value	127,437	166,159
Short-term investments
Fair Value Measurement Inputs Disclosure [Line Items]
Estimated Fair Value	88,553	178,013
U.S. Treasury bills and government agency securities \| Level 2
Fair Value Measurement Inputs Disclosure [Line Items]
Amortized Cost	65,168	110,270
Unrealized Gains	25	1
Unrealized (Losses)		(81)
Estimated Fair Value	$ 65,193	$ 110,190

Fair value measurement inputs disclosure.

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- Definition

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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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
-Topic 320
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