v3.26.1
Stock-Based Compensation - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 3 Months Ended
Jan. 01, 2026
May 23, 2024
Apr. 30, 2021
Mar. 31, 2026
Mar. 31, 2025
Dec. 31, 2025
Feb. 29, 2024
Jul. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Share-based compensation arrangement by share-based payment award, terms of award       The 2018 Amended Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock awards, restricted stock units, unrestricted stock awards, cash-based awards and dividend equivalent rights. The 2018 Amended Plan is administered by either the board of directors or the compensation committee of the board of directors. The exercise prices, vesting and other restrictions are determined at the discretion of the administrator, except that the term of stock options and stock appreciation rights may not be greater than ten years (or five years for certain incentive stock options). Awards typically vest over 12 months to four years. The exercise price for stock options granted may not be less than the fair value of common stock as of the date of grant (or 110% of the fair value of common stock for certain incentive stock options). The fair value of common stock is based on quoted market prices.The 2018 Amended Plan also provides for the assumption of shares remaining available for delivery under the 2019 Stock Plan (as defined below) pursuant to the terms of the Reverse Merger, and such shares will be available for the granting of awards under the 2018 Amended Plan in accordance with applicable stock exchange requirements. The Company also has outstanding stock options under the Magenta Therapeutics, Inc. 2016 Stock Option and Grant Plan, as amended (the “2016 Plan”), but is no longer granting awards under the 2016 Plan.        
Weighted average fair value of stock options granted       $ 39.81        
Number of shares, options vested and expected to vest       2,061,596        
Additional number of shares reserved and available for issuance 2,161,025              
Number of Shares, Granted       1,926,500        
Warrant [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Common stock exercise price     $ 1.65          
Warrants issued     4,677          
Warrant per grant date fair value     $ 1.16          
Stock Options and Stock Appreciation Rights [Member] | Maximum [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Term of award       10 years        
2018 Stock Option and Incentive Plan [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Total unrecognized stock-based compensation expense related to unvested share-based awards       $ 123.1        
Period for recognition of unrecognized expense       3 years 2 months 12 days        
Number of shares reserved and available for issuance increment percentage   4.00%   5.00%        
2018 Amended Plan [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Shares available for issuance       1,546,733        
Percentage of purchase price of common stock under the ESPP       110.00%        
Number of shares reserved and available for issuance increment percentage       5.00%        
Additional number of shares reserved and available for issuance   2,931,820            
2018 Amended Plan [Member] | Stock Options and Stock Appreciation Rights [Member] | Minimum [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Vesting period       12 months        
2018 Amended Plan [Member] | Stock Options and Stock Appreciation Rights [Member] | Maximum [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Vesting period       4 years        
2018 Amended Plan [Member] | Certain Incentive Stock Options [Member] | Maximum [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Term of award       5 years        
2019 Employee Stock Purchase Plan [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Share-based compensation arrangement by share-based payment award, terms of award       The ESPP provides that the number of shares reserved and available for issuance under the ESPP will automatically increase each January 1 through January 1, 2029, by the lesser of (i) 1% of the number of shares issued and outstanding on the immediately preceding December 31, (ii) 62,500 shares and (iii) such number of shares as determined by the Company’s board of directors or its appointed administrator. The number of shares reserved for issuance under the ESPP increased by 62,500 on January 1, 2026.        
Shares available for issuance       197,888        
Percentage of purchase price of common stock under the ESPP       85.00%        
Common stock purchased       7,889 6,938      
Additional number of shares reserved and available for issuance 62,500              
2019 Employee Stock Purchase Plan [Member] | Minimum [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Number of shares reserved and available for issuance increment percentage       1.00%        
Additional number of shares reserved and available for issuance       62,500        
2019 Stock Plan [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Number of shares, options vested and expected to vest               1,273,454
Inducement Plan [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Share-based compensation arrangement by share-based payment award, number of shares authorized           900,000    
Inducement Plan [Member] | Maximum [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Share-based compensation arrangement by share-based payment award, number of shares authorized             300,000  
Amended Inducement Plan [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Share-based compensation arrangement by share-based payment award, number of shares authorized       688,000