Employee Benefits, Net Periodic Benefit Costs and Charges Relating To All Other Employee Benefit Plans(Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Salaried VEBA:
Total net periodic postretirement benefit cost relating to Salaried VEBA	$ 1.1	$ 0.8
Gain on removal of Union VEBA net assets – Note 5	(1.3)	(0.1)
Multiemployer pension plans	1.1	1.1
Total other employee benefit plans	6.0	5.9
Fabricated Products
Salaried VEBA:
Deferred compensation plan	0.1	0.0
Defined contribution plans	4.1	3.6
Total other employee benefit plans	5.3	4.7
All Other
Salaried VEBA:
Deferred compensation plan	0.4	0.1
Defined contribution plans	0.5	0.4
Total other employee benefit plans	0.7	1.2
VEBAs | All Other
Salaried VEBA:
Service cost	0.0	0.0
Interest cost	0.7	0.7
Expected return on plan assets	(1.0)	(1.0)
Amortization of prior service cost	1.2	1.0
Amortization of net actuarial loss	0.2	0.1
Total net periodic postretirement benefit cost relating to Salaried VEBA	$ 1.1	$ 0.8