Segment Information, Financial Information by Reporting Segment Table (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Net sales:
Net Sales	$ 336.4	$ 338.9	$ 1,075.3	$ 1,018.1
Segment operating income (loss):
Operating income (loss)	40.5	32.6	(381.1)	111.1
Interest expense	(4.9)	(9.7)	(19.9)	(27.7)
Other income (expense), net	(0.9)	2.2	(0.1)	5.9
Income (loss) before income taxes	34.7	25.1	(401.1)	89.3
Non-cash net periodic postretirement benefit cost (income) relating to VEBAs1	0.6	(6.0)	1.8	(17.7)
Depreciation and amortization:
Depreciation and amortization	8.1	8.0	24.2	23.1
Capital expenditures:
Capital expenditures	15.5	9.5	38.4	39.6
Income taxes paid:
Income taxes paid	0.3	1.6	1.9	2.7
Assets:
Assets	1,263.1		1,263.1		$ 1,743.7
United States
Income taxes paid:
Income taxes paid	0.1	1.4	0.4	1.6
Canada
Income taxes paid:
Income taxes paid	0.2	0.2	1.5	1.1
Fabricated Products
Net sales:
Net Sales	336.4	338.9	1,075.3	1,018.1
Segment operating income (loss):
Operating income (loss)	47.3	35.6	142.8	121.2
Depreciation and amortization:
Depreciation and amortization	7.9	7.8	23.8	22.7
Capital expenditures:
Capital expenditures	15.1	9.3	37.8	38.8
Assets:
Assets	897.3		897.3		878.9
All Other
Segment operating income (loss):
Operating income (loss)	(6.8)	(3.0)	(523.9)	(10.1)
Depreciation and amortization:
Depreciation and amortization	0.2	0.2	0.4	0.4
Capital expenditures:
Capital expenditures	0.4	0.2	0.6	0.8
Assets:
Assets	365.8		365.8		$ 864.8
VEBAs
Segment operating income (loss):
Non-cash net periodic postretirement benefit cost (income) relating to VEBAs1	0.6	(6.0)	1.8	(17.7)
Pre-tax loss on settlement of Union VEBA	$ (2.9)	$ 0.0	$ 490.9	$ 0.0