Postretirement Benefits Other Than Pensions - Benefit Payments Net, Modernization Act Subsidy (Details) - Other Postretirement Benefit Plan, Defined Benefit [Member] $ in Millions	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Excluding Medicare Part D Subsidy Year One	$ 1.6
Expected Medicare Part D Subsidy, Year One	0.0
Net OPEB Benefits Year One	1.6
Excluding Medicare Part D Subsidy, Year Two	1.6
Expected Medicare Part D Subsidy, Year Two	0.0
Net OPEB Benefits Year Two	1.6
Excluding Medicare Part D Subsidy, Year Three	1.5
Expected Medicare Part D Subsidy, Year Three	0.0
Net OPEB Benefits Year Three	1.5
Excluding Medicare Part D Subsidy, Year Four	1.5
Expected Medicare Part D Subsidy, Year Four	0.0
Net OPEB Benefits Year Four	1.5
Excluding Medicare Part D Subsidy, Year Five	1.4
Expected Medicare Part D Subsidy, Year Five	0.0
Net OPEB Benefits Year Five	1.4
Excluding Medicare Part D Subsidy Fiscal Years Thereafter	5.7
Expected Medicare Part D Subsidy, Five Fiscal Years Thereafter	0.0
Net OPEB Benefits Five Fiscal Years Thereafter	$ 5.7