Pension and Postretirement Benefits - Plan Assets at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 1,250.2	$ 1,696.4
Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	795.7	985.3
Total | Short-term investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9.1	16.7
Total | Total Aon Collective Investment Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	786.6	968.6
Total | Equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	192.6	390.7
Total | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3.0	0.6
Total | Fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	591.0	577.3
Quoted prices in active markets for identical assets (Level I)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	795.7	985.3
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	9.1	16.7
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	786.6	968.6
Quoted prices in active markets for identical assets (Level I) | Equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	192.6	390.7
Quoted prices in active markets for identical assets (Level I) | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3.0	0.6
Quoted prices in active markets for identical assets (Level I) | Fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	591.0	577.3
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	123.4	159.1
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	331.1	552.0
Other Investments Measured at Net Asset Value | Fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	99.8	155.1
Other Investments Measured at Net Asset Value | Venture Capital Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7.0	5.3
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	$ 224.3	$ 391.6