Employee Benefits, Net Periodic Benefit Costs and Charges Relating To All Other Employee Benefit Plans(Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Gain on removal of Union VEBA net assets	$ 0.0	$ 0.0	$ 0.0	$ (1.3)
Total net periodic postretirement benefit cost relating to Salaried VEBA			3.0	2.2
Total other employee benefit plans	4.9	4.2	12.0	10.2
Fabricated Products
Defined Benefit Plan Disclosure [Line Items]
Deferred compensation plan	0.4	0.1	0.4	0.2
Defined contribution plans	1.3	1.4	5.1	5.5
Multiemployer pension plans	1.1	1.2	2.3	2.3
Total other employee benefit plans	2.8	2.7	7.8	8.0
All Other
Defined Benefit Plan Disclosure [Line Items]
Deferred compensation plan	0.5	0.3	0.6	0.7
Defined contribution plans	0.0	0.1	0.5	0.6
Total other employee benefit plans	2.1	1.5	4.2	2.2
VEBAs | All Other
Defined Benefit Plan Disclosure [Line Items]
Gain on removal of Union VEBA net assets	0.0	0.0	0.0	(1.3)
Total net periodic postretirement benefit cost relating to Salaried VEBA	1.6	1.1	3.1	2.2
Salaried VEBA:
Interest cost	0.7	0.8	1.4	1.5
Expected return on plan assets	(0.7)	(1.1)	(1.4)	(2.1)
Amortization of prior service cost	1.4	1.2	2.7	2.4
Amortization of net actuarial loss	$ 0.2	$ 0.2	$ 0.4	$ 0.4