Employee Benefits Employee Benefits, Components of net periodic benefit cost (income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 4.8	$ 3.7	$ 2.8
VEBAs
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost	0.0	0.0
Defined Benefit Plan, Interest Cost	3.0	2.9
All Other | VEBAs
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost	0.0	0.0	0.0
Defined Benefit Plan, Interest Cost	3.0	2.9	2.7
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	4.1	4.1	4.3
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	4.7	4.1	3.0
Defined Benefit Plan, Amortization of Gain (Loss)	(0.9)	(0.5)	(1.0)
Net periodic benefit cost	4.5	3.4	2.4
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)	0.0	0.0	(0.1)
Net periodic benefit cost	$ 0.3	$ 0.3	$ 0.4