Segment and Geographical Area Information, Summary of financial information by operating segment (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net Sales:
Net sales		$ 316.6	$ 336.4	$ 367.2	$ 371.7	$ 338.0	$ 338.9	$ 344.1	$ 335.1	$ 1,391.9	$ 1,356.1	$ 1,297.5
Segment Operating Income (Loss):
Total operating income (loss)		35.2	40.5	37.0	(458.6)	26.8	$ 32.6	$ 46.4	$ 32.1	(345.9)	137.9	173.3
Interest expense										(24.1)	(37.5)	(35.7)
Other income, net										(1.8)	6.7	5.6
Income before income taxes										(371.8)	107.1	143.2
Depreciation and Amortization:
Depreciation and amortization										32.4	31.1	28.1
Capital expenditures:
Capital expenditures										63.1	59.4	70.4
Segment assets:
Total		1,250.1				1,743.7				1,250.1	1,743.7
Non-cash impairment charges										0.1	1.5	0.0
Lower of cost or market inventory write-down		2.6	$ 0.0	$ 0.0	$ 0.0					2.6	0.0	0.0
Unrealized Gain (Loss) on Derivatives										(3.4)	(6.8)	3.9
Non-cash defined benefit net periodic benefit cost (income)	[1]									2.8	(23.5)	(22.0)
Loss on removal of Union VEBA net assets – Note 6										493.4	0.0	0.0
Fabricated Products
Net Sales:
Net sales										1,391.9	1,356.1	1,297.5
Segment Operating Income (Loss):
Total operating income (loss)										190.8	151.4	188.6
Depreciation and Amortization:
Depreciation and amortization										31.9	30.6	27.6
Capital expenditures:
Capital expenditures										62.4	58.5	69.8
Segment assets:
Total		904.8				878.9				904.8	878.9
Additional accruals										1.7	1.2	4.0
Unrealized Gain (Loss) on Derivatives										(3.4)	(10.4)	0.7
All Other
Segment Operating Income (Loss):
Total operating income (loss)										(536.7)	(13.5)	(15.3)
Depreciation and Amortization:
Depreciation and amortization										0.5	0.5	0.5
Capital expenditures:
Capital expenditures										0.7	0.9	0.6
Segment assets:
Total		$ 345.3				$ 864.8				345.3	864.8
Idled Asset [Member] | Fabricated Products
Segment assets:
Non-cash impairment charges										0.1	1.5	0.0
VEBAs | All Other
Segment assets:
Non-cash defined benefit net periodic benefit cost (income)										$ 2.4	$ (23.7)	$ (22.5)

[1] See Note 6 for the impact of removing the Union VEBA (defined in Note 6) net assets.