Nature of the Business - Additional Information (Details)				1 Months Ended	3 Months Ended	12 Months Ended
	Jan. 10, 2025 USD ($)	Jan. 09, 2025 $ / shares	Nov. 07, 2024 USD ($)	Jan. 31, 2024	Mar. 31, 2025 USD ($)	Dec. 31, 2024 USD ($) $ / shares	Dec. 31, 2023 USD ($) $ / shares	May 15, 2023 $ / shares
Common stock, par value | $ / shares						$ 0.0001	$ 0.0001	$ 0.0001
Percentage of workforce reduction				95.00%
Net losses attributable to common stockholders						$ (58,769,000)	$ (190,418,000)
Accumulated deficit						(714,962,000)	$ (656,193,000)
Cash, cash equivalents and short-term investments						$ 118,300,000
Merger Agreement | Kalaris Therapeutics, Inc.
Termination fee required to pay			$ 3,480,000
Termination fee required to receive			10,410,000
Forecast
Percentage of common stock shares outstanding					20.00%
Forecast | AlloVir
Percentage of common stock shares outstanding					20.00%
Net cash at closing of the merger					$ 100,000,000
Equity own percentage					24.66%
Valuation					$ 116,000,000
Forecast | Kalaris Therapeutics, Inc.
Equity own percentage					75.34%
Valuation					$ 347,000,000
Forecast | Merger Agreement | Kalaris Therapeutics, Inc.
Termination fee required to pay					3,480,000
Termination fee required to receive					10,410,000
Minimum | Forecast | AlloVir
Net cash at closing of the merger					1,000,000
Maximum | Merger Agreement | AlloVir
Required to pay costs and expenses			580,000
Maximum | Merger Agreement | Kalaris Therapeutics, Inc.
Required to pay costs and expenses			$ 580,000
Maximum | Forecast
Additional permitted financing					15,000,000
Maximum | Forecast | AlloVir
Net cash at closing of the merger					95,000,000
Additional permitted financing					7,500,000
Maximum | Forecast | Kalaris Therapeutics, Inc.
Additional permitted financing					7,500,000
Maximum | Forecast | Merger Agreement | AlloVir
Required to pay costs and expenses					580,000
Maximum | Forecast | Merger Agreement | Kalaris Therapeutics, Inc.
Required to pay costs and expenses					$ 580,000
Subsequent Event
Reverse stock split conversion ratio		0.043478261
Reverse stock split		On January 9, 2025, the board of directors (the “Board”) of AlloVir, Inc. determined to effect a 1-for-23 reverse stock split of AlloVir’s common stock, par value $0.0001 per share.
Common stock, par value | $ / shares		$ 0.0001
Subsequent Event | Additional Permitted Bridge Financing | Kalaris Therapeutics, Inc.
Debt instrument, issued, principal amount	$ 3,750,000
Subsequent Event | Additional Permitted Financing | AlloVir
Remaining receive an additional permitted financing	3,750,000
Subsequent Event | Additional Permitted Financing | Kalaris Therapeutics, Inc.
Receive an additional permitted financing	7,500,000
Remaining receive an additional permitted financing	3,750,000
Subsequent Event | Convertible Promissory Note
Debt instrument, issued, principal amount	3,750,000
Subsequent Event | Convertible Promissory Note | Kalaris Therapeutics, Inc.
Debt instrument, issued, principal amount	3,750,000
Subsequent Event | Convertible Promissory Note | Additional Permitted Bridge Financing | Kalaris Therapeutics, Inc.
Debt instrument, issued, principal amount	3,750,000
Subsequent Event | Maximum | Convertible Promissory Note
Debt instrument, issued, principal amount	7,500,000
Subsequent Event | Maximum | Convertible Promissory Note | AlloVir
Debt instrument, issued, principal amount	$ 7,500,000