Employee Benefits, Total charges (income) related to all benefit plans (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Loss on removal of Union VEBA net assets – Note 6		$ 493.4	$ 0.0	$ 0.0
Components of Net Periodic Benefit Cost (Income)
Net periodic benefit cost (income)	[1]	2.8	(23.5)	(22.0)
Defined Benefit Plan Net Periodic Benefit Cost And Charges Relating To Other Benefit Plans		505.2	(14.4)	(12.9)
Fabricated Products
Components of Net Periodic Benefit Cost (Income)
Deferred Compensation Arrangement with Individual, Compensation Expense		0.1	0.2	0.3
Defined Contribution Plan, Cost Recognized		7.8	7.3	7.2
Defined Benefit Plan Net Periodic Benefit Cost And Charges Relating To Other Benefit Plans		8.3	7.8	8.0
Fabricated Products | Canadian pension plan
Components of Net Periodic Benefit Cost (Income)
Net periodic benefit cost (income)		0.4	0.3	0.5
All Other
Components of Net Periodic Benefit Cost (Income)
Deferred Compensation Arrangement with Individual, Compensation Expense		0.3	0.7	0.9
Defined Contribution Plan, Cost Recognized		0.8	0.8	0.7
Defined Benefit Plan Net Periodic Benefit Cost And Charges Relating To Other Benefit Plans		496.9	(22.2)	(20.9)
All Other | VEBAs
Components of Net Periodic Benefit Cost (Income)
Net periodic benefit cost (income)		$ 2.4	$ (23.7)	$ (22.5)

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