Segment Information, Financial Information by Reporting Segment Table (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Net sales:
Net Sales	$ 367.2	$ 344.1	$ 738.9	$ 679.2
Segment operating (loss) income:
Operating income (loss)	37.0	46.4	(421.6)	78.5
Interest expense	(5.2)	(9.2)	(15.0)	(18.0)
Other income, net	0.4	1.8	0.8	3.7
(Loss) income before income taxes	32.2	39.0	(435.8)	64.2
Non-cash net periodic postretirement benefit cost (income) relating to VEBAs1	0.6	(6.1)	1.2	(11.7)
Pre-tax loss on settlement of Union VEBA		0.0
Depreciation and amortization:
Depreciation and amortization	8.1	7.7	16.1	15.1
Capital expenditures:
Capital expenditures	11.6	14.7	22.9	30.1
Income taxes paid:
Income taxes paid	0.6	0.4	1.6	1.1
Assets:
Assets	1,251.3		1,251.3		$ 1,743.7
United States
Income taxes paid:
Income taxes paid	0.2	0.1	0.3	0.2
Canada
Income taxes paid:
Income taxes paid	0.4	0.3	1.3	0.9
Fabricated Products
Net sales:
Net Sales	367.2	344.1	738.9	679.2
Segment operating (loss) income:
Operating income (loss)	50.6	50.2	95.5	85.6
Depreciation and amortization:
Depreciation and amortization	8.0	7.7	15.9	14.9
Capital expenditures:
Capital expenditures	11.5	14.2	22.7	29.5
Assets:
Assets	900.1		900.1		878.9
All Other
Segment operating (loss) income:
Operating income (loss)	(13.6)	(3.8)	(517.1)	(7.1)
Depreciation and amortization:
Depreciation and amortization	0.1	0.0	0.2	0.2
Capital expenditures:
Capital expenditures	0.1	0.5	0.2	0.6
Assets:
Assets	351.2		351.2		$ 864.8
VEBAs
Segment operating (loss) income:
Non-cash net periodic postretirement benefit cost (income) relating to VEBAs1	0.6	(6.1)	1.2	(11.7)
Pre-tax loss on settlement of Union VEBA	$ 1.6	$ 0.0	$ 493.8	$ 0.0