Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	3 Months Ended		9 Months Ended	12 Months Ended
	Jan. 01, 2024	Apr. 30, 2021	Mar. 31, 2024	Mar. 31, 2023	Sep. 30, 2023	Dec. 31, 2023	Feb. 29, 2024	Apr. 30, 2020	Jul. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value of stock options granted			$ 14.03
Number of Shares, Options vested and expected to vest as of September 30, 2023			776,110
Number of Shares, Granted			1,400,200
Warrant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock exercise price		$ 1.65
Vesting period					7 years 1 month 6 days
Warrants issued		4,677
Warrant per grant date fair value								$ 1.16
2018 Stock Option and Incentive 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 2018 Incentive 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. In connection with the Reverse Merger, the 2018 Incentive Plan also provided for the assumption of shares remaining available for delivery under the 2019 Stock Plan (as defined below), and such shares will be available for the granting of awards under the 2018 Incentive 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.
Shares available for issuance			171,590
Total unrecognized stock-based compensation expense related to unvested share-based awards			$ 25.2
Period for recognition of unrecognized expense			3 years 3 months 18 days
Percentage of purchase price of common stock under the ESPP			110.00%
Number of shares reserved and available for issuance increment percentage			4.00%
Additional number of shares reserved and available for issuance	592,707
2018 Stock Option and Incentive Plan [Member] | Restricted Stock [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			12 months
2018 Stock Option and Incentive Plan [Member] | Restricted Stock [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			4 years
2018 Stock Option and Incentive Plan [Member] | 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] | 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, 2024.
Shares available for issuance			99,578
Percentage of purchase price of common stock under the ESPP			85.00%
Common stock purchased			0	0
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 as of September 30, 2023									1,273,454
Inducement Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Granted			270,000			96,000
Inducement Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares for issuance under the Plan (shares)							300,000