Pension and Other Post Retirement Benefits - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) Entity Postretirement_Plan	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits			$ 9.0
Number of Postretirement Plans | Postretirement_Plan	2
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 1.9	$ 1.8	1.9
Defined Benefit Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost	6.5	6.1	$ 7.4
Aggregate accumulated benefit obligation	457.0	441.8
Defined benefit plan, estimated future employer contributions in next fiscal year	6.4
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 1.6	$ 2.2
Loan Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of Limited Liability Company with Investments | Entity	1