Employee Benefits - Benefit Obligations and Funded Status of Canadian Pension and Salaried VEBAs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in plan assets:
Fair market value of plan assets at beginning of year	$ 70.6
Fair market value of plan assets at end of year	77.1	$ 70.6
Salaried VEBA
Change in benefit obligation:
Obligation at beginning of year	90.2	85.6
Service cost	0.1	0.1
Interest cost	2.5	3.2
Prior service (credit) cost	(12.3)	2.5
Actuarial loss	4.7	6.4
Benefits paid by Salaried VEBA	(6.9)	(7.6)
Obligation at end of year	78.3	90.2	$ 85.6
Change in plan assets:
Fair market value of plan assets at beginning of year	57.6	53.2
Actual return on assets	8.1	9.1
Company contributions	1.7	2.9
Benefits paid by Salaried VEBA	(6.9)	(7.6)
Fair market value of plan assets at end of year	60.5	57.6	53.2
Net funded status	(17.8)	(32.6)
Canadian pension plan | Canadian pension plan
Change in benefit obligation:
Obligation at beginning of year	8.8	7.3
Foreign currency translation adjustment	0.3	0.3
Service cost	0.3	0.3	0.3
Interest cost	0.3	0.3	0.3
Actuarial loss	0.6	1.1
Benefits paid by Company	(0.3)	(0.5)
Obligation at end of year	10.0	8.8	7.3
Change in plan assets:
Fair market value of plan assets at beginning of year	7.8	6.5
Foreign currency translation adjustment	0.2	0.3
Actual return on assets	0.7	1.0
Company contributions	0.3	0.5
Benefits paid by Company	(0.3)	(0.5)
Fair market value of plan assets at end of year	8.7	7.8	$ 6.5
Net funded status	$ (1.3)	$ (1.0)