Employee Benefits, Amount of loss recognized in accumulated other comprehensive loss (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Salaried VEBA
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Gain (Loss), before Tax	$ (12.1)	$ (12.5)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, Prior Service Cost (Credit), before Tax	(41.1)	(44.2)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax	(53.2)	(56.7)
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, before Tax	$ (1.9)	$ (1.5)