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Employee Benefits - Fair Value of U.S. Pension Plan Assets (Detail) - USD ($)
$ in Millions
Dec. 31, 2022
Dec. 31, 2021
Northern Trust Private Equity Funds    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List] Fair Value Measured at Net Asset Value Per Share [Member] Fair Value Measured at Net Asset Value Per Share [Member]
Northern Trust Hedge Fund    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List] Fair Value Measured at Net Asset Value Per Share [Member] Fair Value Measured at Net Asset Value Per Share [Member]
Real Estate Funds    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List] Fair Value Measured at Net Asset Value Per Share [Member] Fair Value Measured at Net Asset Value Per Share [Member]
Pension Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value $ 1,176.8 $ 1,708.9
Pension Plans, Defined Benefit | Domestic Common Stock    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 14.7 17.5
Pension Plans, Defined Benefit | Foreign Common Stock    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.5 0.3
Pension Plans, Defined Benefit | Domestic Corporate Bonds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 263.6 341.7
Pension Plans, Defined Benefit | Foreign Corporate Bonds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 39.6 60.9
Pension Plans, Defined Benefit | U.S. Government Obligations    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 77.5 103.9
Pension Plans, Defined Benefit | Non-U.S. Government Obligations    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 13.8 20.0
Pension Plans, Defined Benefit | Domestic Municipal and Provincial Bonds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 16.8 21.0
Pension Plans, Defined Benefit | Foreign Municipal and Provincial Bonds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.2 0.3
Pension Plans, Defined Benefit | Collective Trust Funds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 522.3 875.3
Pension Plans, Defined Benefit | Mutual Funds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 61.4 108.9
Pension Plans, Defined Benefit | Cash and Other    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value (1.1) 8.6
Pension Plans, Defined Benefit | Northern Trust Private Equity Funds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 33.2 34.5
Pension Plans, Defined Benefit | Northern Trust Hedge Fund    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 35.2 36.7
Pension Plans, Defined Benefit | Real Estate Funds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 99.1 79.3
Pension Plans, Defined Benefit | Fair Value, Inputs, Level 1, 2 and 3    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 1,009.3 1,558.4
Pension Plans, Defined Benefit | LEVEL 1    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 75.5 135.3
Pension Plans, Defined Benefit | LEVEL 1 | Domestic Common Stock    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 14.7 17.5
Pension Plans, Defined Benefit | LEVEL 1 | Foreign Common Stock    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.5 0.3
Pension Plans, Defined Benefit | LEVEL 1 | Domestic Corporate Bonds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 1 | Foreign Corporate Bonds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 1 | U.S. Government Obligations    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 1 | Non-U.S. Government Obligations    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 1 | Domestic Municipal and Provincial Bonds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 1 | Foreign Municipal and Provincial Bonds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 1 | Collective Trust Funds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 1 | Mutual Funds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 61.4 108.9
Pension Plans, Defined Benefit | LEVEL 1 | Cash and Other    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value (1.1) 8.6
Pension Plans, Defined Benefit | LEVEL 2    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 933.8 1,423.1
Pension Plans, Defined Benefit | LEVEL 2 | Domestic Common Stock    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 2 | Foreign Common Stock    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 2 | Domestic Corporate Bonds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 263.6 341.7
Pension Plans, Defined Benefit | LEVEL 2 | Foreign Corporate Bonds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 39.6 60.9
Pension Plans, Defined Benefit | LEVEL 2 | U.S. Government Obligations    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 77.5 103.9
Pension Plans, Defined Benefit | LEVEL 2 | Non-U.S. Government Obligations    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 13.8 20.0
Pension Plans, Defined Benefit | LEVEL 2 | Domestic Municipal and Provincial Bonds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 16.8 21.0
Pension Plans, Defined Benefit | LEVEL 2 | Foreign Municipal and Provincial Bonds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.2 0.3
Pension Plans, Defined Benefit | LEVEL 2 | Collective Trust Funds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 522.3 875.3
Pension Plans, Defined Benefit | LEVEL 2 | Mutual Funds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 2 | Cash and Other    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 3    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 3 | Domestic Common Stock    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 3 | Foreign Common Stock    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 3 | Domestic Corporate Bonds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 3 | Foreign Corporate Bonds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 3 | U.S. Government Obligations    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 3 | Non-U.S. Government Obligations    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 3 | Domestic Municipal and Provincial Bonds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 3 | Foreign Municipal and Provincial Bonds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 3 | Collective Trust Funds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 3 | Mutual Funds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value 0.0 0.0
Pension Plans, Defined Benefit | LEVEL 3 | Cash and Other    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets at Fair Value $ 0.0 $ 0.0