Pension Benefits - Fair Value Plan Assets Measurements (Details) - Pension Plans, Defined Benefit - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	$ 405.4	$ 664.6	$ 525.3
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	249.7	400.2
Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	82.0	128.9
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0	$ 0.3
United States Government and Government Agencies and Authorities
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	68.3	158.4
United States Government and Government Agencies and Authorities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	68.3	158.4
United States Government and Government Agencies and Authorities | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
United States Government and Government Agencies and Authorities | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
States and Political Subdivisions
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.7	0.1
States and Political Subdivisions | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
States and Political Subdivisions | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.7	0.1
States and Political Subdivisions | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Corporate Bonds and Notes
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	81.3	107.3
Corporate Bonds and Notes | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Corporate Bonds and Notes | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	81.3	107.3
Corporate Bonds and Notes | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Other Industries
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	64.8	161.5
Other Industries | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	64.8	140.0
Other Industries | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	21.5
Other Industries | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Collective Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	32.1	71.8
Collective Investment Funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Collective Investment Funds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Collective Investment Funds | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Bond Exchange Traded Funds
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	108.6	92.8
Bond Exchange Traded Funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	108.6	92.8
Bond Exchange Traded Funds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Bond Exchange Traded Funds | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Limited Liability Companies and Limited Partnerships
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	41.6	63.7
Limited Liability Companies and Limited Partnerships | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Limited Liability Companies and Limited Partnerships | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Limited Liability Companies and Limited Partnerships | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	7.4	10.0
Short-term Investments | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	7.4	10.0
Short-term Investments | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Short-term Investments | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Receivables and Other
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.6	(1.0)
Receivables and Other | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.6	(1.0)
Receivables and Other | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Receivables and Other | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Measured at Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	73.7	135.5
Measured at Net Asset Value | United States Government and Government Agencies and Authorities
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Measured at Net Asset Value | States and Political Subdivisions
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Measured at Net Asset Value | Corporate Bonds and Notes
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Measured at Net Asset Value | Other Industries
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Measured at Net Asset Value | Collective Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	32.1	71.8
Measured at Net Asset Value | Bond Exchange Traded Funds
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Measured at Net Asset Value | Limited Liability Companies and Limited Partnerships
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	41.6	63.7
Measured at Net Asset Value | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.0	0.0
Measured at Net Asset Value | Receivables and Other
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	$ 0.0	$ 0.0