Segment and Geographical Area Information, Financial Information by Reporting Segment Table (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Net Sales:
Net sales	$ 338.0		$ 338.9	$ 344.1	$ 335.1	$ 311.3		$ 319.9	$ 328.9	$ 337.4	$ 1,356.1		$ 1,297.5		$ 1,360.1
Segment Operating Income (Loss):
Total operating income (loss)	26.8		32.6	46.4	32.1	41.6		41.6	40.1	50.0	137.9		173.3		165.9
Interest expense											(37.5)		(35.7)		(29.1)
Other income, net											6.7		5.6		2.8
Income before income taxes											107.1		143.2		139.6
Depreciation and Amortization:
Depreciation and amortization											31.1		28.1		26.5
Capital expenditures:
Capital expenditures											59.4		70.4		44.1
Segment assets:
Total	1,743.7					1,770.9					1,743.7		1,770.9
Fabricated Products
Net Sales:
Net sales											1,356.1		1,297.5		1,360.1
Segment Operating Income (Loss):
Total operating income (loss)											151.4	[1],[2]	188.6	[1],[2]	190.8	[1],[2]
Depreciation and Amortization:
Depreciation and amortization											30.6		27.6		26.0
Capital expenditures:
Capital expenditures											58.5		69.8		43.8
Segment assets:
Total	878.9					852.5					878.9		852.5
All Other
Segment Operating Income (Loss):
Total operating income (loss)											(13.5)	[3]	(15.3)	[3]	(24.9)	[3]
Depreciation and Amortization:
Depreciation and amortization											0.5		0.5		0.5
Capital expenditures:
Capital expenditures											0.9		0.6		0.3
Segment assets:
Total	$ 864.8	[4]				$ 918.4	[4]				$ 864.8	[4]	$ 918.4	[4]

[1] Fabricated Products segment results for 2014, 2013 and 2012 include non-cash mark-to-market (losses) gains on primary aluminum, natural gas, electricity and foreign currency hedging activities totaling $(10.4), $0.7 and $15.2, respectively. For further discussion regarding mark-to-market matters, see Note 10.

[2] Operating results in the Fabricated Products segment for 2014, 2013 and 2012 included $1.2, $4.0 and $1.1, respectively, of environmental expense. Fabricated Products segment operating results for 2014 and 2012 included $1.5 and $4.4 of asset impairment charge relating to certain property, plant and equipment.

[3] Operating results of All Other include VEBA net periodic pension benefit income of $23.7, $22.5 and $11.9 for 2014, 2013 and 2012, respectively.

[4] Assets in All Other represent primarily all of the Company’s cash and cash equivalents, short-term investments, financial derivative assets, net assets of VEBAs and net deferred income tax assets.