Available-for-Sale Marketable Securities - Summary of Available-for-Sale Marketable Securities by Major Type of Security (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 55,731	$ 199,690
Gross Unrealized Gains	363	197
Gross Unrealized Losses		(27)
Estimated Fair Value	56,094	199,860
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,081	16,052
Gross Unrealized Gains	98	31
Gross Unrealized Losses		(2)
Estimated Fair Value	9,179	16,081
U.S. Government Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,499	25,803
Gross Unrealized Gains	8	14
Gross Unrealized Losses		(1)
Estimated Fair Value	3,507	25,816
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	41,827	115,788
Gross Unrealized Gains	256	125
Gross Unrealized Losses		(23)
Estimated Fair Value	42,083	115,890
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,324	38,547
Gross Unrealized Gains	1	27
Gross Unrealized Losses		(1)
Estimated Fair Value	$ 1,325	38,573
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		3,500
Estimated Fair Value		$ 3,500