Pensions and Other Postretirement Benefits - Fair Value of Plan Assets (Details) - Pension Benefit Plans - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 1,172.4	$ 1,186.5	$ 1,340.7
Minimum
Defined Benefit Plan Disclosure [Line Items]
Asset allocation horizon term	3 years
Maximum
Defined Benefit Plan Disclosure [Line Items]
Asset allocation horizon term	5 years
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 29.0	24.2
Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,130.2	1,156.5
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13.2	5.8	$ 0.8
Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	159.6	58.8
Cash | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.3	0.3
Cash | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	159.3	58.5
Cash | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Equity Securities, Domestic
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	82.9	86.4
Equity Securities, Domestic | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4.7	3.6
Equity Securities, Domestic | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	78.2	82.8
Equity Securities, Domestic | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Equity Securities, Foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7.0	9.2
Equity Securities, Foreign | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7.0	5.3
Equity Securities, Foreign | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	3.8
Equity Securities, Foreign | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	852.5	980.1
Fixed Income Securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	17.0	15.0
Fixed Income Securities | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	835.3	962.3
Fixed Income Securities | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.2	2.8
Real Estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21.9	9.2
Real Estate | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Real Estate | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8.9	7.6
Real Estate | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13.0	1.6
Diversified Growth Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	48.5	42.8
Diversified Growth Fund | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Diversified Growth Fund | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	48.5	41.5
Diversified Growth Fund | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0.0	$ 1.4