Employee Benefits Employee Benefits, Amount of loss recognized in accumulated other comprehensive loss (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Salaried VEBA
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Gain (Loss), before Tax	$ (17.5)	$ (18.3)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, Prior Service Cost (Credit), before Tax	42.7	40.2
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax	60.2	58.5
Canadian pension plan | Canadian pension plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Gain (Loss), before Tax	(1.9)	(1.5)
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Transition Asset (Obligation), before Tax	0.1	0.1
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax	$ 1.8	$ 1.4