Retirement Benefits - Summary of Company's Plan Assets, Benefit Obligations, and Funded Status (Footnote) (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Pension benefit obligation, non-qualified plan	$ 19.0	$ 18.8
Medicare Part D Subsidy [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefits paid for medical and life benefits	0.0	0.1
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Benefits paid for medical and life benefits	$ 10.6	$ 134.1