Employee Benefits - Fair Value of U.S. Pension Plan Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
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	$ 1,708.9	$ 1,793.7
Pension Plans, Defined Benefit | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	17.5	13.5
Pension Plans, Defined Benefit | Foreign Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.3	314.4
Pension Plans, Defined Benefit | Domestic Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	341.7
Pension Plans, Defined Benefit | Foreign Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	60.9	43.6
Pension Plans, Defined Benefit | U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	103.9	116.0
Pension Plans, Defined Benefit | Non-U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	20.0	22.9
Pension Plans, Defined Benefit | Domestic Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	21.0	22.4
Pension Plans, Defined Benefit | Foreign Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.3	0.3
Pension Plans, Defined Benefit | Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	837.2	985.5
Pension Plans, Defined Benefit | Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	108.9	167.0
Pension Plans, Defined Benefit | Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	46.7	7.5
Pension Plans, Defined Benefit | Northern Trust Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	34.5	20.3
Pension Plans, Defined Benefit | Northern Trust Hedge Fund
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	36.7	34.2
Pension Plans, Defined Benefit | Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	79.3	46.1
Pension Plans, Defined Benefit | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,558.4	1,693.1
Pension Plans, Defined Benefit | LEVEL 1
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	173.4	190.9
Pension Plans, Defined Benefit | LEVEL 1 | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	17.5	13.5
Pension Plans, Defined Benefit | LEVEL 1 | Foreign Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.3	0.0
Pension Plans, Defined Benefit | LEVEL 1 | Domestic Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	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	2.9
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	108.9	167.0
Pension Plans, Defined Benefit | LEVEL 1 | Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	46.7	7.5
Pension Plans, Defined Benefit | LEVEL 2
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,385.0	1,502.2
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	314.4
Pension Plans, Defined Benefit | LEVEL 2 | Domestic Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	341.7
Pension Plans, Defined Benefit | LEVEL 2 | Foreign Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	60.9	43.6
Pension Plans, Defined Benefit | LEVEL 2 | U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	103.9	113.1
Pension Plans, Defined Benefit | LEVEL 2 | Non-U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	20.0	22.9
Pension Plans, Defined Benefit | LEVEL 2 | Domestic Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	21.0	22.4
Pension Plans, Defined Benefit | LEVEL 2 | Foreign Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.3	0.3
Pension Plans, Defined Benefit | LEVEL 2 | Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	837.2	985.5
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
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