Pension and Postretirement Benefits - Plan Assets at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 1,282.7	$ 1,250.2
Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	871.7	795.7
Total | Short-term investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	23.4	9.1
Total | Total Aon Collective Investment Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	848.3	786.6
Total | Equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	222.3	192.6
Total | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3.3	3.0
Total | Fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	622.7	591.0
Quoted prices in active markets for identical assets (Level I)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	871.7	795.7
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	23.4	9.1
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	848.3	786.6
Quoted prices in active markets for identical assets (Level I) | Equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	222.3	192.6
Quoted prices in active markets for identical assets (Level I) | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3.3	3.0
Quoted prices in active markets for identical assets (Level I) | Fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	622.7	591.0
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	92.4	123.4
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	318.6	331.1
Other Investments Measured at Net Asset Value | Fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	76.3	99.8
Other Investments Measured at Net Asset Value | Venture Capital Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6.6	7.0
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	$ 235.7	$ 224.3