Pension and Postretirement Benefits - Plan Assets at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 1,604.2	$ 1,572.9
Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	951.8	1,108.4
Total | Short-term investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21.2	16.1
Total | Aon Hewitt Collective Trust Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	930.6	1,092.3
Total | Equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	448.5	450.8
Total | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6.8	76.1
Total | Fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	475.3	565.4
Quoted Prices in Active Markets for Identical Assets (Level I)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	951.8	1,108.4
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	21.2	16.1
Quoted Prices in Active Markets for Identical Assets (Level I) | Aon Hewitt Collective Trust Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	930.6	1,092.3
Quoted Prices in Active Markets for Identical Assets (Level I) | Equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	448.5	450.8
Quoted Prices in Active Markets for Identical Assets (Level I) | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6.8	76.1
Quoted Prices in Active Markets for Identical Assets (Level I) | Fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	475.3	565.4
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) | Aon Hewitt Collective Trust Investment 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) | Aon Hewitt Collective Trust Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value Measured at Net Asset Value Per Share | Aon Hewitt Collective Trust Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	147.5
Fair Value Measured at Net Asset Value Per Share | Other Investments Measured At Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	504.9	464.5
Fair Value Measured at Net Asset Value Per Share | Venture Capital Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4.7	3.9
Fair Value Measured at Net Asset Value Per Share | Other Non-U.S. commingled equity and fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	362.9	333.2
Fair Value Measured at Net Asset Value Per Share | Fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 137.3	$ 127.4