Benefit Plans and Other Postretirement Benefits, Fair value measurements of plan assets (Details) - Pension Benefits - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure
Fair value of pension plan assets	$ 497.9	$ 439.9	$ 392.2
U.S. equities — large cap
Defined Benefit Plan Disclosure
Fair value of pension plan assets	141.5	118.8
U.S. equities — small/mid cap and other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	22.0	25.8
International equities — growth
Defined Benefit Plan Disclosure
Fair value of pension plan assets	35.6	46.5
International equities — other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	95.5	60.6
Alternative investment funds
Defined Benefit Plan Disclosure
Fair value of pension plan assets	12.9	12.6
U.S. fixed income securities — short term
Defined Benefit Plan Disclosure
Fair value of pension plan assets	6.1	6.0
U.S. fixed income securities — intermediate term
Defined Benefit Plan Disclosure
Fair value of pension plan assets	61.4	58.4
U.S. fixed income securities — high yield
Defined Benefit Plan Disclosure
Fair value of pension plan assets	21.9	22.2
International fixed income securities — other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	47.4	43.0
Insurance contracts
Defined Benefit Plan Disclosure
Fair value of pension plan assets	37.9	34.0
Real estate funds
Defined Benefit Plan Disclosure
Fair value of pension plan assets	7.0
Cash and cash equivalents
Defined Benefit Plan Disclosure
Fair value of pension plan assets	8.7	12.0
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure
Fair value of pension plan assets	207.1	212.6
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	106.6	88.7
Quoted Prices in Active Markets for Identical Assets (Level 1) | International equities — growth
Defined Benefit Plan Disclosure
Fair value of pension plan assets	30.4	46.5
Quoted Prices in Active Markets for Identical Assets (Level 1) | International equities — other
Defined Benefit Plan Disclosure
Fair value of pension plan assets		7.0
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	61.4	58.4
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Defined Benefit Plan Disclosure
Fair value of pension plan assets	8.7	12.0
Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure
Fair value of pension plan assets	172.1	155.0
Significant Observable Inputs (Level 2) | U.S. equities — large cap
Defined Benefit Plan Disclosure
Fair value of pension plan assets	34.9	30.1
Significant Observable Inputs (Level 2) | U.S. equities — small/mid cap and other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	22.0	25.8
Significant Observable Inputs (Level 2) | International equities — growth
Defined Benefit Plan Disclosure
Fair value of pension plan assets	5.2
Significant Observable Inputs (Level 2) | International equities — other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	40.7	33.9
Significant Observable Inputs (Level 2) | U.S. fixed income securities — high yield
Defined Benefit Plan Disclosure
Fair value of pension plan assets	21.9	22.2
Significant Observable Inputs (Level 2) | International fixed income securities — other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	47.4	43.0
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure
Fair value of pension plan assets	44.9	34.0
Significant Unobservable Inputs (Level 3) | Insurance contracts
Defined Benefit Plan Disclosure
Fair value of pension plan assets	37.9	34.0
Significant Unobservable Inputs (Level 3) | Real estate funds
Defined Benefit Plan Disclosure
Fair value of pension plan assets	7.0
Fair Value Measured At Net Asset Value
Defined Benefit Plan Disclosure
Fair value of pension plan assets	73.8	38.3
Fair Value Measured At Net Asset Value | International equities — other
Defined Benefit Plan Disclosure
Fair value of pension plan assets	54.8	19.7
Fair Value Measured At Net Asset Value | Alternative investment funds
Defined Benefit Plan Disclosure
Fair value of pension plan assets	12.9	12.6
Fair Value Measured At Net Asset Value | U.S. fixed income securities — short term
Defined Benefit Plan Disclosure
Fair value of pension plan assets	$ 6.1	$ 6.0