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	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 404,432	$ 296,133
Unrealized Gain	57	2
Unrealized (Loss)	(21)	(286)
Estimated Fair Value	404,468	295,849
Cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	153,453	117,837
Unrealized Gain	0	1
Unrealized (Loss)	0	(2)
Estimated Fair Value	153,453	117,836
Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	214,029	178,296
Unrealized Gain	35	1
Unrealized (Loss)	(10)	(284)
Estimated Fair Value	214,054	178,013
Long-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	36,950	0
Unrealized Gain	22	0
Unrealized (Loss)	(11)	0
Estimated Fair Value	36,961	0
Fair Value, Inputs, Level 1 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	23,826	19,500
Unrealized Gain	0	0
Unrealized (Loss)	0	0
Estimated Fair Value	23,826	19,500
Fair Value, Inputs, Level 2 | Corporate notes and commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	284,410	166,363
Unrealized Gain	9	1
Unrealized (Loss)	(20)	(205)
Estimated Fair Value	284,399	166,159
Fair Value, Inputs, Level 2 | U.S. Treasury bills and government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	96,196	110,270
Unrealized Gain	48	1
Unrealized (Loss)	(1)	(81)
Estimated Fair Value	$ 96,243	$ 110,190