Pension and Postretirement Benefits - Plan Assets at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 1,696.4	$ 1,620.4
Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	985.3	951.8
Total | Short-term investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16.7	21.2
Total | Total Aon Collective Investment Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	968.6	930.6
Total | Equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	390.7	448.5
Total | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.6	6.8
Total | Fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	577.3	475.3
Quoted prices in active markets for identical assets (Level I)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	985.3	951.8
Quoted prices in active markets for identical assets (Level I) | Short-term investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16.7	21.2
Quoted prices in active markets for identical assets (Level I) | Total Aon Collective Investment Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	968.6	930.6
Quoted prices in active markets for identical assets (Level I) | Equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	390.7	448.5
Quoted prices in active markets for identical assets (Level I) | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.6	6.8
Quoted prices in active markets for identical assets (Level I) | Fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	577.3	475.3
Significant other observable inputs (Level II)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant other observable inputs (Level II) | Short-term investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant other observable inputs (Level II) | Total Aon Collective Investment Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant other observable inputs (Level II) | Equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant other observable inputs (Level II) | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant other observable inputs (Level II) | Fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant unobservable inputs (Level III)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant unobservable inputs (Level III) | Short-term investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant unobservable inputs (Level III) | Total Aon Collective Investment Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant unobservable inputs (Level III) | Equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant unobservable inputs (Level III) | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant unobservable inputs (Level III) | Fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Other Investments Measured at Net Asset Value | Total Aon Collective Investment Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	159.1	147.5
Other Investments Measured at Net Asset Value | Total other investments measured at net asset value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	552.0	521.1
Other Investments Measured at Net Asset Value | Fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	155.1	137.3
Other Investments Measured at Net Asset Value | Venture Capital Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5.3	4.7
Other Investments Measured at Net Asset Value | Other Non-U.S. commingled equity and fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 391.6	$ 379.1