Postretirement Benefits Other Than Pensions - Benefit Payments Net, Modernization Act Subsidy (Details) (Other Postretirement Benefit Plan, Defined Benefit [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Excluding Medicare Part D Subsidy Year One	$ 3.6
Expected Medicare Part D Subsidy, Year One	(0.4)
Net OPEB Benefits Year One	3.2
Excluding Medicare Part D Subsidy, Year Two	3.7
Expected Medicare Part D Subsidy, Year Two	(0.4)
Net OPEB Benefits Year Two	3.3
Excluding Medicare Part D Subsidy, Year Three	3.6
Expected Medicare Part D Subsidy, Year Three	(0.4)
Net OPEB Benefits Year Three	3.2
Excluding Medicare Part D Subsidy, Year Four	3.5
Expected Medicare Part D Subsidy, Year Four	(0.4)
Net OPEB Benefits Year Four	3.1
Excluding Medicare Part D Subsidy, Year Five	3.4
Expected Medicare Part D Subsidy, Year Five	(0.4)
Net OPEB Benefits Year Five	3.0
Excluding Medicare Part D Subsidy Fiscal Years Thereafter	13.8
Expected Medicare Part D Subsidy, Five Fiscal Years Thereafter	(1.6)
Net OPEB Benefits Five Fiscal Years Thereafter	$ 12.2