Other Postretirement Employee Benefit Plans (Benefit Payments That Reflect Expected Future Service) (Details) - Other postretirement benefits $ in Millions	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Expected future benefit payments, next 12 months	$ 15.8
Expected future benefit payments, in year two	15.2
Expected future benefit payments, in year three	14.6
Expected future benefit payments, in year four	14.0
Expected future benefit payments, in year five	13.3
Expected future benefit payments, thereafter	$ 57.8