Stock-Based Compensation - Additional Information (Details) - shares	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Jan. 01, 2025	Jan. 01, 2024
Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award Line Items
Options granted in replacement of RSU	8,608	1,365,343
2018 Plan
Share Based Compensation Arrangement By Share Based Payment Award Line Items
Increase in number of shares			2,385,497	2,347,245
Outstanding stock including non-voting common stock, percent	4.00%
Common stock available for future issuance	7,924,642
2018 Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award Line Items
Increase in number of shares	8,768,800

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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+ Details

IGM Biosciences, Inc. 10-Q