Retirement Benefits - Summary of Company's Plan Assets, Benefit Obligations, and Funded Status (Footnote) (Details) $ in Millions		12 Months Ended
	Oct. 17, 2017 USD ($) Participant	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Pension benefit obligation		$ 18.9	$ 18.8
Medicare Part D Subsidy
Defined Benefit Plan Disclosure [Line Items]
Benefit paid during period		0.1	0.1
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Benefit paid during period		133.2	134.1
amount of non-participating annuity contracts acquired	$ 34.7
Number of define benefit plan participants | Participant	2,800
Pension benefit obligation		1,442.9	$ 1,492.1	$ 1,531.0
Payment for pension benefits		$ 42.3