Retirement Benefits - Schedule of Fair Value of Pension Plan Assets and Liabilities by Asset Category and by Level (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 605.5	$ 571.1
Receivables	0.8	6.5
Payables	(18.6)	(20.1)
Total assets	957.0	932.5
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0.1
Domestic equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	406.3	372.6
International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	37.0	33.5
Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	71.5	78.2
Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	28.7	26.3
U.S. government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	41.4	31.3
Municipal bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.6
Foreign bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.5	0.8
Foreign exchange contracts
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0.1
Registered investment companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	16.4	22.8
Private assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	3.1	5.4
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	453.6	424.8
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0.1
Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	400.2	368.4
Quoted Prices in Active Markets for Identical Assets (Level 1) | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	37.0	33.5
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign exchange contracts
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Registered investment companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	16.4	22.8
Quoted Prices in Active Markets for Identical Assets (Level 1) | Private assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	110.8	109.1
Other Observable Inputs (Level 2) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0.0
Other Observable Inputs (Level 2) | Domestic equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Other Observable Inputs (Level 2) | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Other Observable Inputs (Level 2) | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	39.6	50.6
Other Observable Inputs (Level 2) | Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	28.7	26.3
Other Observable Inputs (Level 2) | U.S. government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	41.4	31.3
Other Observable Inputs (Level 2) | Municipal bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.6
Other Observable Inputs (Level 2) | Foreign bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.5	0.8
Other Observable Inputs (Level 2) | Foreign exchange contracts
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0.1
Other Observable Inputs (Level 2) | Registered investment companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Other Observable Inputs (Level 2) | Private assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	41.1	37.2
Total assets	41.1	37.2	$ 27.8
Unobservable Inputs (Level 3) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0.0
Unobservable Inputs (Level 3) | Domestic equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	6.1	4.2
Total assets	6.1	4.2	4.9
Unobservable Inputs (Level 3) | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Total assets		0.0	0.1
Unobservable Inputs (Level 3) | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	31.9	27.6
Total assets	31.9	27.6	21.5
Unobservable Inputs (Level 3) | Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Unobservable Inputs (Level 3) | U.S. government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Unobservable Inputs (Level 3) | Municipal bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0
Unobservable Inputs (Level 3) | Foreign bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Unobservable Inputs (Level 3) | Foreign exchange contracts
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0.0
Unobservable Inputs (Level 3) | Registered investment companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Unobservable Inputs (Level 3) | Private assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	3.1	5.4
Total assets	3.1	5.4	$ 1.3
NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	369.3	375.0
NAV | Private assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	100.8	106.3
NAV | Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	232.3	218.7
NAV | CCTs
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 36.2	$ 50.0