Employee Benefits, Total charges (income) related to all benefit plans (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
(Gain) loss on removal of Union VEBA net assets – Note 6		$ (0.1)	$ 493.4	$ 0.0
Components of Net Periodic Benefit Cost (Income)
Net periodic benefit cost (income)	[1]	3.7	2.8	(23.5)
Contributions of the Company		4.7	4.4	4.0
Defined Benefit Plan Net Periodic Benefit Cost And Charges Relating To Other Benefit Plans		18.1	509.6	(10.4)
Fabricated Products
Components of Net Periodic Benefit Cost (Income)
Deferred Compensation Arrangement with Individual, Compensation Expense		0.2	0.1	0.2
Defined Contribution Plan, Cost Recognized		8.1	7.8	7.3
Defined Benefit Plan Net Periodic Benefit Cost And Charges Relating To Other Benefit Plans		13.3	12.7	11.8
Fabricated Products | Canadian pension plan
Components of Net Periodic Benefit Cost (Income)
Net periodic benefit cost (income)		0.3	0.4	0.3
All Other
Components of Net Periodic Benefit Cost (Income)
Deferred Compensation Arrangement with Individual, Compensation Expense		0.7	0.3	0.7
Defined Contribution Plan, Cost Recognized		0.8	0.8	0.8
Defined Benefit Plan Net Periodic Benefit Cost And Charges Relating To Other Benefit Plans		4.8	496.9	(22.2)
All Other | VEBAs
Components of Net Periodic Benefit Cost (Income)
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.