Employee Benefits - Benefit Obligations and Funded Status of Canadian Pension and Salaried VEBAs (Parenthetical) (Details) - Salaried VEBA - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss due to change in discount rate	$ 5.7	$ 6.8
Actuarial loss due to changes in census data	$ (1.0)	0.1
Actuarial gain due to change in the projected utilization rate		$ (0.5)