Fair Value of Financial Assets and Liabilities, Allocated into Level 1, Level 2, and Level 3 Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value Measurement Inputs Disclosure [Line Items]
Amortized Cost	$ 287,680	$ 296,133
Unrealized Gains	5	2
Unrealized (Losses)	(86)	(286)
Estimated Fair Value	287,599	295,849
Money market funds \| Level 1
Fair Value Measurement Inputs Disclosure [Line Items]
Amortized Cost	34,054	19,500
Estimated Fair Value	34,054	19,500
Cash equivalents
Fair Value Measurement Inputs Disclosure [Line Items]
Estimated Fair Value	191,551	117,836
Corporate notes and commercial paper \| Level 2
Fair Value Measurement Inputs Disclosure [Line Items]
Amortized Cost	168,707	166,363
Unrealized Gains	2	1
Unrealized (Losses)	(77)	(205)
Estimated Fair Value	168,632	166,159
Short-term investments
Fair Value Measurement Inputs Disclosure [Line Items]
Estimated Fair Value	96,048	178,013
U.S. Treasury bills and government agency securities \| Level 2
Fair Value Measurement Inputs Disclosure [Line Items]
Amortized Cost	84,919	110,270
Unrealized Gains	3	1
Unrealized (Losses)	(9)	(81)
Estimated Fair Value	$ 84,913	$ 110,190

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).

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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).

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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).

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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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