Retirement Plans and Profit Sharing Arrangements - Pension Plan Assets Target Allocations (Details) - Pension Plans	12 Months Ended
	Dec. 31, 2021
Maximum
Defined Benefit Plan, Assets, Target Allocations
Actual allocations within target allocations (as a percent)	5.00%
Pension plans, excluding Threadneedle and BMO Financial Group plans | Equity securities
Defined Benefit Plan, Assets, Target Allocations
Target allocations (as a percent)	70.00%
Pension plans, excluding Threadneedle and BMO Financial Group plans | Debt securities
Defined Benefit Plan, Assets, Target Allocations
Target allocations (as a percent)	20.00%
Pension plans, excluding Threadneedle and BMO Financial Group plans | All other types of investments
Defined Benefit Plan, Assets, Target Allocations
Target allocations (as a percent)	10.00%
Threadneedle pension plans | Pooled equity securities
Defined Benefit Plan, Assets, Target Allocations
Target allocations (as a percent)	70.00%
Threadneedle pension plans | Pooled debt securities
Defined Benefit Plan, Assets, Target Allocations
Target allocations (as a percent)	30.00%
BMO Financial Group pension plans | Collective funds
Defined Benefit Plan, Assets, Target Allocations
Target allocations (as a percent)	90.00%
BMO Financial Group pension plans | Hedge fund investments
Defined Benefit Plan, Assets, Target Allocations
Target allocations (as a percent)	10.00%