Retirement Plans - Fair Value of Pension Assets (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pension and other postretirement benefit asset	$ 22.6	$ 19.2
Cash and short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pension and other postretirement benefit asset	0.5	0.4
Equity funds and other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pension and other postretirement benefit asset	2.7	2.3
Fixed income mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pension and other postretirement benefit asset	19.4	16.5
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pension and other postretirement benefit asset	4.6	3.8
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pension and other postretirement benefit asset	0.5	0.4
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity funds and other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pension and other postretirement benefit asset	2.7	2.3
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pension and other postretirement benefit asset	1.4	1.1
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pension and other postretirement benefit asset	18.0	15.4
Significant Other Observable Inputs (Level 2) | Fixed income mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pension and other postretirement benefit asset	18.0	15.4
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pension and other postretirement benefit asset	$ 0.0	$ 0.0