Benefit Plans and Other Postretirement Benefits, Fair value measurements of plan assets (Details) - Pension Benefits - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure
Fair value of pension plan assets	$ 514.1	$ 497.9	$ 439.9
U.S. equities — large cap
Defined Benefit Plan Disclosure
Fair value of pension plan assets	139.3	141.5
U.S. equities — small/mid cap and other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	27.5	22.0
International equities — growth
Defined Benefit Plan Disclosure
Fair value of pension plan assets	32.7	35.6
International equities — other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	84.9	95.5
Alternative investment funds
Defined Benefit Plan Disclosure
Fair value of pension plan assets	19.9	12.9
U.S. fixed income securities — short term
Defined Benefit Plan Disclosure
Fair value of pension plan assets	11.2	6.1
U.S. fixed income securities — intermediate term
Defined Benefit Plan Disclosure
Fair value of pension plan assets	77.7	61.4
U.S. fixed income securities — high yield
Defined Benefit Plan Disclosure
Fair value of pension plan assets	26.4	21.9
International fixed income securities — other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	45.9	47.4
Insurance contracts
Defined Benefit Plan Disclosure
Fair value of pension plan assets	35.1	37.9
Real estate funds
Defined Benefit Plan Disclosure
Fair value of pension plan assets	10.2	7.0
Cash and cash equivalents
Defined Benefit Plan Disclosure
Fair value of pension plan assets	3.3	8.7
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure
Fair value of pension plan assets	213.9	207.1
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	104.9	106.6
Quoted Prices in Active Markets for Identical Assets (Level 1) | International equities — growth
Defined Benefit Plan Disclosure
Fair value of pension plan assets	28.0	30.4
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	77.7	61.4
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Defined Benefit Plan Disclosure
Fair value of pension plan assets	3.3	8.7
Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure
Fair value of pension plan assets	172.9	172.1
Significant Observable Inputs (Level 2) | U.S. equities — large cap
Defined Benefit Plan Disclosure
Fair value of pension plan assets	34.4	34.9
Significant Observable Inputs (Level 2) | U.S. equities — small/mid cap and other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	27.5	22.0
Significant Observable Inputs (Level 2) | International equities — growth
Defined Benefit Plan Disclosure
Fair value of pension plan assets	4.7	5.2
Significant Observable Inputs (Level 2) | International equities — other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	34.0	40.7
Significant Observable Inputs (Level 2) | U.S. fixed income securities — high yield
Defined Benefit Plan Disclosure
Fair value of pension plan assets	26.4	21.9
Significant Observable Inputs (Level 2) | International fixed income securities — other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	45.9	47.4
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure
Fair value of pension plan assets	45.3	44.9	$ 34.0
Significant Unobservable Inputs (Level 3) | Insurance contracts
Defined Benefit Plan Disclosure
Fair value of pension plan assets	35.1	37.9
Significant Unobservable Inputs (Level 3) | Real estate funds
Defined Benefit Plan Disclosure
Fair value of pension plan assets	10.2	7.0
Fair Value Measured At Net Asset Value
Defined Benefit Plan Disclosure
Fair value of pension plan assets	82.0	73.8
Fair Value Measured At Net Asset Value | International equities — other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	50.9	54.8
Fair Value Measured At Net Asset Value | Alternative investment funds
Defined Benefit Plan Disclosure
Fair value of pension plan assets	19.9	12.9
Fair Value Measured At Net Asset Value | U.S. fixed income securities — short term
Defined Benefit Plan Disclosure
Fair value of pension plan assets	$ 11.2	$ 6.1