Segment Information, Financial Information by Reporting Segment Table (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013		Dec. 31, 2013
Net sales:
Net Sales	$ 338.9		$ 319.9		$ 1,018.1		$ 986.2
Segment operating income (loss):
Operating income (loss)	32.6		41.6		111.1		131.7
Interest expense	(9.7)		(8.8)		(27.7)		(27.1)
Other income, net	2.2		2.9		5.9		3.1
Income before income taxes	25.1		35.7		89.3		107.7
Depreciation and amortization:
Depreciation and amortization	8.0		6.9		23.1		20.9
Capital expenditures:
Capital expenditures	9.5		21.0		39.6		47.0
Income taxes paid:
Income taxes paid	1.6		0.5		2.7		1.8
Assets:
Assets	1,816.3				1,816.3				1,770.9
Fabricated Products
Net sales:
Net Sales	338.9		319.9		1,018.1		986.2
Segment operating income (loss):
Operating income (loss)	35.6	[1]	44.8	[1]	121.2	[1]	145.0	[1]
Depreciation and amortization:
Depreciation and amortization	7.8		6.8		22.7		20.6
Capital expenditures:
Capital expenditures	9.3		21.0		38.8		46.4
Assets:
Assets	871.3				871.3				852.5
Fabricated Products | United States
Income taxes paid:
Income taxes paid	1.4		0.3		1.6		0.9
Fabricated Products | Canada
Income taxes paid:
Income taxes paid	0.2		0.2		1.1		0.9
All Other
Segment operating income (loss):
Operating income (loss)	(3.0)	[2]	(3.2)	[2]	(10.1)	[2]	(13.3)	[2]
Depreciation and amortization:
Depreciation and amortization	0.2		0.1		0.4		0.3
Capital expenditures:
Capital expenditures	0.2		0		0.8		0.6
Assets:
Assets	$ 945.0	[3]			$ 945.0	[3]			$ 918.4	[3]

[1] Fabricated Products segment operating income included non-cash mark-to-market (losses) gains on primary aluminum, natural gas, electricity and foreign currency hedging activities totaling $(3.5) and $1.5 for the quarters ended September 30, 2014 and September 30, 2013, respectively. Non-cash mark-to-market gains (losses) on primary aluminum, natural gas, electricity and foreign currency hedging activities totaled $0.0 and $(3.4) for the nine months ended September 30, 2014 and September 30, 2013, respectively. For further discussion regarding mark-to-market matters, see Note 8.

[2] Operating loss in All Other included VEBA net periodic pension benefit income of $6.0 and $5.6 for the quarters ended September 30, 2014 and September 30, 2013, respectively. VEBA net periodic pension benefit income was $17.7 and $16.9 for the nine months ended September 30, 2014 and September 30, 2013, respectively.

[3] Assets in All Other represent primarily all of the Company’s cash and cash equivalents, short-term investments, financial derivative assets, net assets in respect of VEBA(s) (see Note 5) and net deferred income tax assets.