Stockholders' Equity - Narrative (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended				9 Months Ended
	Sep. 30, 2024	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023	Dec. 31, 2021	Dec. 31, 2020
Class of Stock [Line Items]
Common stock, voting rights						one
Total intrinsic value of options exercised		$ 0.1			$ 0.7	$ 0.4	$ 2.5
Cost not yet recognized	$ 0.2	0.2				$ 0.2
Stock compensation not yet recognized, vesting period						8 months 12 days
September 2024 Workforce Reduction
Class of Stock [Line Items]
Restructuring charges	0.6
Employee severance, inclusive of cash expenditures for employee separation costs | September 2024 Workforce Reduction
Class of Stock [Line Items]
Restructuring charges	0.5
Non-cash charges, stock based compensation | September 2024 Workforce Reduction
Class of Stock [Line Items]
Restructuring charges	$ 0.1
General and administrative expense
Class of Stock [Line Items]
Share-based payment arrangement expense		1.8			1.7	$ 5.9	4.7
Research and development expense
Class of Stock [Line Items]
Share-based payment arrangement expense		$ 0.7			0.7	$ 1.8	2.2
Stock Plans
Class of Stock [Line Items]
Stock option plan, exercisable period						4 years
Stock Plans | Share-based payment arrangement, nonemployee
Class of Stock [Line Items]
Stock option plan, exercisable period						10 years
Stock Plans | One year from the vesting commencement date
Class of Stock [Line Items]
Stock option plan, vesting percentage						25.00%
Option vesting period						1 year
Stock Plans | More than one year from vesting commencement date
Class of Stock [Line Items]
Stock option plan, vesting percentage						2.778%
Employee Stock | Employee Stock Purchase Plan
Class of Stock [Line Items]
Purchase price of common stock						85.00%
Number of shares authorized (in shares)								5,639,944		1,846,710
Evergreen shares reserve feature period						10 years
Increase in shares authorized						1.00%
Increase in number of shares of authorized common stock (in shares)						3,693,420
Employee Stock | Stock Plans
Class of Stock [Line Items]
Common stock, shares reserved (in shares)	30,040,211	30,040,211				30,040,211		28,761,816
Stock options available for grant (in shares)	10,468,774	10,468,774				10,468,774
Employee Stock | 2021 Equity Incentive Plan
Class of Stock [Line Items]
Number of shares authorized (in shares)									18,467,109
Evergreen shares reserve feature period						10 years
Increase in shares authorized						5.00%
Maximum percentage of share reserve, outstanding diluted stock						15.00%
Number of additional shares authorized (in shares)						1,278,395
RSU awards
Class of Stock [Line Items]
Option vesting period						1 year 3 months 18 days
Fair value of options vested		$ 1.0			$ 0.7	$ 2.4	$ 1.3
Number of RSU awards vested (in shares)						720,514
Issuance of common shares (in shares)						134,064
Vested (in shares)		167,768	206,049	212,633
RSU awards | Stock Plans
Class of Stock [Line Items]
Option vesting period						3 years
Requisite service period						3 years
RSU awards | Stock Plans | Tranche two
Class of Stock [Line Items]
Stock option plan, vesting percentage						33.30%
RSU awards | Stock Plans | Tranche three
Class of Stock [Line Items]
Stock option plan, vesting percentage						33.30%
RSU awards | Stock Plans | Tranche one
Class of Stock [Line Items]
Stock option plan, vesting percentage						33.30%
RSU awards | 2021 Equity Incentive Plan
Class of Stock [Line Items]
Stock compensation not yet recognized, vesting period						3 years
Cost not yet recognized, excluding options	$ 10.6	$ 10.6				$ 10.6
Performance-based stock awards | Stock Plans | Minimum
Class of Stock [Line Items]
Stock option plan, vesting percentage						0.00%
Performance-based stock awards | Stock Plans | Maximum
Class of Stock [Line Items]
Stock option plan, vesting percentage						300.00%
Performance-based stock awards | 2021 Equity Incentive Plan
Class of Stock [Line Items]
Vested (in shares)		25,200				79,200
Performance-based stock
Class of Stock [Line Items]
Shares issued (in shares)						455,368
Share-based payment arrangement expense						$ 9.2