Employee Benefits, Net Periodic Benefit Costs and Charges Relating To All Other Employee Benefit Plans(Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Salaried VEBA1:
Total net periodic postretirement benefit cost relating to Salaried VEBA	$ 1.2	$ 0.8	$ 3.4	$ 2.5
Gain on removal of Union VEBA net assets	0.5	0.0	(0.8)	(0.1)
Total other employee benefit plans	4.7	4.2	14.9	14.1
Fabricated Products
Salaried VEBA1:
Deferred compensation plan	0.1	0.1	0.3	0.2
Defined contribution plans	1.3	1.6	6.8	6.7
Multiemployer pension plans	1.1	1.2	3.4	3.5
Total other employee benefit plans	2.5	2.9	10.5	10.4
All Other
Salaried VEBA1:
Deferred compensation plan	0.4	0.4	1.1	0.6
Defined contribution plans	0.1	0.1	0.7	0.7
Total other employee benefit plans	2.2	1.3	4.4	3.7
VEBAs | All Other
Salaried VEBA1:
Total net periodic postretirement benefit cost relating to Salaried VEBA	1.2	0.8	3.4	2.5
Gain on removal of Union VEBA net assets	0.5	0.0	(0.8)	(0.1)
Voluntary Employees' beneficiary Association (VEBA) | VEBAs | All Other
Salaried VEBA1:
Interest cost	0.8	0.7	2.3	2.2
Expected return on plan assets	(1.0)	(1.0)	(3.1)	(3.0)
Amortization of prior service cost	1.2	1.0	3.6	3.0
Amortization of net actuarial loss	0.2	0.1	0.6	0.3
Total net periodic postretirement benefit cost relating to Salaried VEBA	$ 1.2	$ 0.8	$ 3.4	$ 2.5