Summary of Available-for-Sale Securities (Detail) - USD ($)	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 87,890,000	$ 102,529,000
Gross Unrealized Gain	185,000	677,000
Gross Unrealized Loss	(4,000)	(3,000)
Allowance for Credit Loss	0	0	$ 0
Fair Value	88,071,000	103,203,000
Money market funds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	46,570,000	6,203,000
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Allowance for Credit Loss	0	0
Fair Value	46,570,000	6,203,000
United States government agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,023,000	29,570,000
Gross Unrealized Gain	2,000	66,000
Gross Unrealized Loss	0	0
Allowance for Credit Loss	0	0
Fair Value	7,025,000	29,636,000
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	34,297,000	66,756,000
Gross Unrealized Gain	183,000	611,000
Gross Unrealized Loss	(4,000)	(3,000)
Allowance for Credit Loss	0	0
Fair Value	$ 34,476,000	$ 67,364,000