Benefit Plans and Other Postretirement Benefits, Fair value measurements of plan assets (Details) - Pension Benefits - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure
Fair value of pension plan assets	$ 581.0	$ 514.1	$ 497.9
U.S. equities - large cap
Defined Benefit Plan Disclosure
Fair value of pension plan assets	112.0	139.3
U.S. equities - small/mid cap and other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	36.4	27.5
International equities - growth
Defined Benefit Plan Disclosure
Fair value of pension plan assets	56.9	32.7
International equities - other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	74.9	84.9
Alternative investment funds
Defined Benefit Plan Disclosure
Fair value of pension plan assets	21.1	19.9
U.S. fixed income securities - short term
Defined Benefit Plan Disclosure
Fair value of pension plan assets	18.1	11.2
U.S. fixed income securities - intermediate term
Defined Benefit Plan Disclosure
Fair value of pension plan assets	26.8	77.7
Fixed Income Securities - Long Term [Member]
Defined Benefit Plan Disclosure
Fair value of pension plan assets	118.6
U.S. fixed income securities - high yield
Defined Benefit Plan Disclosure
Fair value of pension plan assets	14.2	26.4
International fixed income securities - other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	24.0	45.9
Insurance contracts
Defined Benefit Plan Disclosure
Fair value of pension plan assets	37.0	35.1
Real estate funds
Defined Benefit Plan Disclosure
Fair value of pension plan assets	10.7	10.2
Cash and cash equivalents
Defined Benefit Plan Disclosure
Fair value of pension plan assets	30.3	3.3
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure
Fair value of pension plan assets	165.8	213.9
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. equities - large cap
Defined Benefit Plan Disclosure
Fair value of pension plan assets	56.9	104.9
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. equities - small/mid cap and other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | International equities - growth
Defined Benefit Plan Disclosure
Fair value of pension plan assets	51.8	28.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | International equities - other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Alternative investment funds
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. fixed income securities - short term
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. fixed income securities - intermediate term
Defined Benefit Plan Disclosure
Fair value of pension plan assets	26.8	77.7
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed Income Securities - Long Term [Member]
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. fixed income securities - high yield
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | International fixed income securities - other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Insurance contracts
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Real estate funds
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Defined Benefit Plan Disclosure
Fair value of pension plan assets	30.3	3.3
Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure
Fair value of pension plan assets	312.5	172.9
Significant Observable Inputs (Level 2) | U.S. equities - large cap
Defined Benefit Plan Disclosure
Fair value of pension plan assets	55.1	34.4
Significant Observable Inputs (Level 2) | U.S. equities - small/mid cap and other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	36.4	27.5
Significant Observable Inputs (Level 2) | International equities - growth
Defined Benefit Plan Disclosure
Fair value of pension plan assets	5.1	4.7
Significant Observable Inputs (Level 2) | International equities - other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	41.0	34.0
Significant Observable Inputs (Level 2) | Alternative investment funds
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Significant Observable Inputs (Level 2) | U.S. fixed income securities - short term
Defined Benefit Plan Disclosure
Fair value of pension plan assets	18.1	0.0
Significant Observable Inputs (Level 2) | U.S. fixed income securities - intermediate term
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Significant Observable Inputs (Level 2) | Fixed Income Securities - Long Term [Member]
Defined Benefit Plan Disclosure
Fair value of pension plan assets	118.6
Significant Observable Inputs (Level 2) | U.S. fixed income securities - high yield
Defined Benefit Plan Disclosure
Fair value of pension plan assets	14.2	26.4
Significant Observable Inputs (Level 2) | International fixed income securities - other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	24.0	45.9
Significant Observable Inputs (Level 2) | Insurance contracts
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Significant Observable Inputs (Level 2) | Real estate funds
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Significant Observable Inputs (Level 2) | Cash and cash equivalents
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure
Fair value of pension plan assets	47.7	45.3	$ 44.9
Significant Unobservable Inputs (Level 3) | U.S. equities - large cap
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | U.S. equities - small/mid cap and other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | International equities - growth
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | International equities - other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | Alternative investment funds
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | U.S. fixed income securities - short term
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | U.S. fixed income securities - intermediate term
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | Fixed Income Securities - Long Term [Member]
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0
Significant Unobservable Inputs (Level 3) | U.S. fixed income securities - high yield
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | International fixed income securities - other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | Insurance contracts
Defined Benefit Plan Disclosure
Fair value of pension plan assets	37.0	35.1
Significant Unobservable Inputs (Level 3) | Real estate funds
Defined Benefit Plan Disclosure
Fair value of pension plan assets	10.7	10.2
Significant Unobservable Inputs (Level 3) | Cash and cash equivalents
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Fair Value Measured At Net Asset Value
Defined Benefit Plan Disclosure
Fair value of pension plan assets	55.0	82.0
Fair Value Measured At Net Asset Value | U.S. equities - large cap
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Fair Value Measured At Net Asset Value | U.S. equities - small/mid cap and other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Fair Value Measured At Net Asset Value | International equities - growth
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Fair Value Measured At Net Asset Value | International equities - other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	33.9	50.9
Fair Value Measured At Net Asset Value | Alternative investment funds
Defined Benefit Plan Disclosure
Fair value of pension plan assets	21.1	19.9
Fair Value Measured At Net Asset Value | U.S. fixed income securities - short term
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	11.2
Fair Value Measured At Net Asset Value | U.S. fixed income securities - intermediate term
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Fair Value Measured At Net Asset Value | Fixed Income Securities - Long Term [Member]
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0
Fair Value Measured At Net Asset Value | U.S. fixed income securities - high yield
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Fair Value Measured At Net Asset Value | International fixed income securities - other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Fair Value Measured At Net Asset Value | Insurance contracts
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Fair Value Measured At Net Asset Value | Real estate funds
Defined Benefit Plan Disclosure
Fair value of pension plan assets	0.0	0.0
Fair Value Measured At Net Asset Value | Cash and cash equivalents
Defined Benefit Plan Disclosure
Fair value of pension plan assets	$ 0.0	$ 0.0