Summary of Significant Accounting Policies - Marketable Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	$ 113,991	$ 166,076
Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	42,518	18,597
Estimated Fair Value	42,518	18,597
Cash and Cash Equivalents [Member] | Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	42,518	18,597
Estimated Fair Value	42,518	18,597
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	113,909	165,996
Gross Unrealized Gains	82	80
Estimated Fair Value	113,991	166,076
Short-term Investments [Member] | U.S. and Foreign Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	31,059	51,102
Estimated Fair Value	31,059	51,102
Short-term Investments [Member] | U.S. and Foreign Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	25,773	56,691
Gross Unrealized Gains	57	38
Estimated Fair Value	25,830	56,729
Short-term Investments [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,600	39,756
Gross Unrealized Gains	5	33
Estimated Fair Value	9,605	39,789
Short-term Investments [Member] | U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	31,984	18,447
Gross Unrealized Gains	18	9
Estimated Fair Value	32,002	18,456
Short-term Investments [Member] | US Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,493
Gross Unrealized Gains	2
Maketable Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	156,427	184,593
Gross Unrealized Gains	82	80
Estimated Fair Value	$ 156,509	$ 184,673