Pension and Other Post-Retirement Benefits - Pension Obligations, Plan Assets, and Other Post-retirement Obligations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Accumulative other comprehensive loss:
After-tax amount recognized in accumulated other comprehensive loss	$ 98.1		$ 83.6
Defined Benefit Pension [Member]
Change in Benefit Obligation
Service cost	1.9	$ 1.6
Interest cost	3.7	3.8
Retiree Health and Life [Member]
Change in Benefit Obligation
Service cost	0.1	0.0
Interest cost	$ 0.2	$ 0.2