Employee Benefits, Amount of loss recognized in accumulated other comprehensive loss (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Salaried VEBA
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Gain (Loss), before Tax	$ (12.5)	$ (17.5)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, Prior Service Cost (Credit), before Tax	44.2	42.7
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax	56.7	60.2
Canadian pension plan | Canadian pension plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Gain (Loss), before Tax	(1.5)	(1.9)
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Transition Asset (Obligation), before Tax	0.0	0.1
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax	$ 1.5	$ 1.8