Employee Benefits, Components of net periodic benefit cost (income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Net periodic postretirement benefit cost	$ 7.0	$ 6.6	$ 4.8
VEBAs
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost	0.1	0.1	0.0
Defined Benefit Plan, Interest Cost	3.2	2.7	3.0
Expected return on plan assets	(2.7)	(2.9)	(4.1)
Amortization of prior service cost	5.6	5.4	4.7
Amortization of net actuarial loss	0.4	0.8	0.9
Net periodic postretirement benefit cost	6.6	6.1	4.5
Canadian pension plan | Canadian pension plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost	0.3	0.3	0.3
Defined Benefit Plan, Interest Cost	0.3	0.3	0.3
Expected return on plan assets	(0.3)	(0.3)	(0.3)
Amortization of prior service cost	0.0	0.0	0.0
Amortization of net actuarial loss	0.1	0.1	0.0
Net periodic postretirement benefit cost	$ 0.4	$ 0.4	$ 0.3