Employee Benefits, Components of net periodic benefit cost (income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Net periodic postretirement benefit cost	$ 6.6	$ 4.8	$ 3.7
VEBAs
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost	0.0	0.0
Defined Benefit Plan, Interest Cost	2.8	3.0
Net periodic postretirement benefit cost	6.1	4.5	3.4
All Other | VEBAs
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost	0.1	0.0	0.0
Defined Benefit Plan, Interest Cost	2.7	3.0	2.9
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	2.9	4.1	4.1
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	5.4	4.7	4.1
Defined Benefit Plan, Amortization of Gain (Loss)	(0.8)	(0.9)	(0.5)
Net periodic postretirement benefit cost	6.1	4.5	3.4
Canadian pension plan | Canadian pension plan
Defined Benefit Plan Disclosure [Line Items]
Net periodic postretirement benefit cost	0.4	0.3	0.3
Canadian pension plan | Fabricated Products | 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
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	0.3	0.3	0.3
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	0.0	0.0	0.0
Defined Benefit Plan, Amortization of Gain (Loss)	(0.1)	0.0	0.0
Net periodic postretirement benefit cost	$ 0.4	$ 0.3	$ 0.3