Employee Benefits, Total charges (income) related to all benefit plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
(Gain) loss on removal of Union VEBA net assets – Note 4	$ 0.0	$ (0.1)	$ 493.4
Components of Net Periodic Benefit Cost (Income)
Net periodic benefit cost	4.8	3.7	2.8
Contributions of the Company	4.6	4.7	4.4
Defined Benefit Plan Net Periodic Benefit Cost And Charges Relating To Other Benefit Plans	20.1	18.1	509.6
Fabricated Products
Components of Net Periodic Benefit Cost (Income)
Deferred Compensation Arrangement with Individual, Compensation Expense	0.4	0.2	0.1
Defined Contribution Plan, Cost	8.1	8.1	7.8
Defined Benefit Plan Net Periodic Benefit Cost And Charges Relating To Other Benefit Plans	13.4	13.3	12.7
All Other
Components of Net Periodic Benefit Cost (Income)
Deferred Compensation Arrangement with Individual, Compensation Expense	1.4	0.7	0.3
Defined Contribution Plan, Cost	0.8	0.8	0.8
Defined Benefit Plan Net Periodic Benefit Cost And Charges Relating To Other Benefit Plans	6.7	4.8	496.9
All Other | VEBAs
Components of Net Periodic Benefit Cost (Income)
Net periodic benefit cost	4.5	3.4	2.4
Canadian pension plan | Fabricated Products | Canadian pension plan
Components of Net Periodic Benefit Cost (Income)
Net periodic benefit cost	$ 0.3	$ 0.3	$ 0.4