Employee Benefits - Fair Value of U.S. Pension Plan Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
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 Plan Disclosure [Line Items]
Plan Assets at Fair Value	$ 1,338.1	$ 1,200.8
Pension Plans | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	17.6	16.6
Pension Plans | Foreign Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.3	0.3
Pension Plans | Domestic Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	244.9	218.2
Pension Plans | Foreign Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	30.1	33.6
Pension Plans | U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	58.3	87.5
Pension Plans | Non-U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	17.0	14.0
Pension Plans | Domestic Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	14.3	17.2
Pension Plans | Foreign Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.2	0.3
Pension Plans | Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	734.9	571.5
Pension Plans | Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	56.3	80.3
Pension Plans | Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	(1.2)	(3.4)
Pension Plans | Northern Trust Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	53.2	45.6
Pension Plans | Northern Trust Hedge Fund
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	39.2	36.2
Pension Plans | Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	73.0	82.9
Pension Plans | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,172.7	1,036.1
Pension Plans | LEVEL 1
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	131.3	181.3
Pension Plans | LEVEL 1 | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	17.6	16.6
Pension Plans | LEVEL 1 | Foreign Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.3	0.3
Pension Plans | LEVEL 1 | Domestic Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 1 | Foreign Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 1 | U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	58.3	87.5
Pension Plans | LEVEL 1 | Non-U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 1 | Domestic Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 1 | Foreign Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 1 | Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 1 | Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	56.3	80.3
Pension Plans | LEVEL 1 | Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	(1.2)	(3.4)
Pension Plans | LEVEL 2
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,041.4	854.8
Pension Plans | LEVEL 2 | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 2 | Foreign Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 2 | Domestic Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	244.9	218.2
Pension Plans | LEVEL 2 | Foreign Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	30.1	33.6
Pension Plans | LEVEL 2 | U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 2 | Non-U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	17.0	14.0
Pension Plans | LEVEL 2 | Domestic Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	14.3	17.2
Pension Plans | LEVEL 2 | Foreign Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.2	0.3
Pension Plans | LEVEL 2 | Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	734.9	571.5
Pension Plans | LEVEL 2 | Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 2 | Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 3
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 3 | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 3 | Foreign Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 3 | Domestic Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 3 | Foreign Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 3 | U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 3 | Non-U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 3 | Domestic Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 3 | Foreign Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 3 | Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 3 | Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans | LEVEL 3 | Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	$ 0.0	$ 0.0