Employee Benefits Employee Benefits, Components of net periodic benefit cost (income) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (income)	[1]	$ 3.7	$ 2.8	$ (23.5)
VEBAs
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost		0.0	0.0
Defined Benefit Plan, Interest Cost		2.9	2.7
Fabricated Products | Canadian pension plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost		0.3	0.3	0.2
Defined Benefit Plan, Interest Cost		0.3	0.3	0.3
Defined Benefit Plan, Expected Return 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 Gains (Losses)		0.0	(0.1)	(0.1)
Net periodic benefit cost (income)		0.3	0.4	0.3
All Other | VEBAs
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost		0.0	0.0	2.2
Defined Benefit Plan, Interest Cost		2.9	2.7	16.7
Defined Benefit Plan, Expected Return on Plan Assets		4.1	4.3	51.4
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)		4.1	3.0	10.6
Defined Benefit Plan, Amortization of Gains (Losses)		(0.5)	(1.0)	1.8
Net periodic benefit cost (income)		$ 3.4	$ 2.4	$ (23.7)

[1] See Note 6 for the impact of removing the Union VEBA (defined in Note 6) net assets.