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	Mar. 31, 2020	Dec. 31, 2019
Fair Value Measurement Inputs Disclosure [Line Items]
Amortized Cost	$ 346,191	$ 404,432
Unrealized Gain	672	57
Unrealized (Loss)	(76)	(21)
Estimated Fair Value	346,787	404,468
Money market funds | Level 1
Fair Value Measurement Inputs Disclosure [Line Items]
Amortized Cost	54,735	23,826
Unrealized Gain	0	0
Unrealized (Loss)	0	0
Estimated Fair Value	54,735	23,826
Corporate notes and commercial paper | Level 2
Fair Value Measurement Inputs Disclosure [Line Items]
Amortized Cost	207,206	284,410
Unrealized Gain	64	9
Unrealized (Loss)	(76)	(20)
Estimated Fair Value	207,194	284,399
U.S. Treasury bills and government agency securities | Level 2
Fair Value Measurement Inputs Disclosure [Line Items]
Amortized Cost	84,250	96,196
Unrealized Gain	608	48
Unrealized (Loss)	0	(1)
Estimated Fair Value	84,858	96,243
Cash equivalents
Fair Value Measurement Inputs Disclosure [Line Items]
Amortized Cost	148,944	153,453
Unrealized Gain	18	0
Unrealized (Loss)	0	0
Estimated Fair Value	148,962	153,453
Short-term investments
Fair Value Measurement Inputs Disclosure [Line Items]
Amortized Cost	166,314	214,029
Unrealized Gain	407	35
Unrealized (Loss)	(60)	(10)
Estimated Fair Value	166,661	214,054
Long-term investments
Fair Value Measurement Inputs Disclosure [Line Items]
Amortized Cost	30,933	36,950
Unrealized Gain	247	22
Unrealized (Loss)	(16)	(11)
Estimated Fair Value	$ 31,164	$ 36,961