Retirement Plans - Allocation of Plan Assets, Pension Plan (Details) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	$ 256.4	$ 235.8	$ 0.0
Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	66.7	71.7
Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	90.9	96.2
Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	7.0	6.4
Cash and Cash Equivalents | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	3.2	4.1
Cash and Cash Equivalents | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	3.8	2.3
Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	63.5	67.6
Equity | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	63.5	67.6
Equity | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	0.0	0.0
Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	87.1	93.9
Fixed Income | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	0.0	0.0
Fixed Income | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	87.1	93.9
Equity Funds Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	98.8	67.9
Equity Funds Measured at NAV | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	0.0	0.0
Equity Funds Measured at NAV | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	$ 0.0	$ 0.0