Segment Information, Financial Information by Reporting Segment Table (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Net sales:
Net Sales	$ 371.7us-gaap_SalesRevenueGoodsNet	$ 335.1us-gaap_SalesRevenueGoodsNet
Segment operating (loss) income:
Operating income (loss)	(458.6)us-gaap_OperatingIncomeLoss	32.1us-gaap_OperatingIncomeLoss
Interest expense	(9.8)us-gaap_InterestExpense	(8.8)us-gaap_InterestExpense
Other income, net	0.4us-gaap_OtherNonoperatingIncomeExpense	1.9us-gaap_OtherNonoperatingIncomeExpense
(Loss) income before income taxes	(468.0)	25.2
Non-cash net periodic postretirement benefit charges (income) relating to VEBAs1	0.6	(5.6)
Pre-tax loss on settlement of Union VEBA		0kalu_PretaxlossonsettlementofUnionVEBA
Depreciation and amortization:
Depreciation and amortization	8.0	7.4
Capital expenditures:
Capital expenditures	11.3	15.4
Income taxes paid:
Income taxes paid	1.0us-gaap_IncomeTaxesPaid	0.7us-gaap_IncomeTaxesPaid
Assets:
Assets	1,534.9us-gaap_Assets		1,743.7us-gaap_Assets
United States
Income taxes paid:
Income taxes paid	0.1	0.1
Canada
Income taxes paid:
Income taxes paid	0.9	0.6
Fabricated Products
Net sales:
Net Sales	371.7	335.1
Segment operating (loss) income:
Operating income (loss)	44.9	35.4
Depreciation and amortization:
Depreciation and amortization	7.9	7.2
Capital expenditures:
Capital expenditures	11.2	15.3
Assets:
Assets	905.9		878.9
All Other
Segment operating (loss) income:
Operating income (loss)	(503.5)	(3.3)
Depreciation and amortization:
Depreciation and amortization	0.1	0.2
Capital expenditures:
Capital expenditures	0.1	0.1
Assets:
Assets	629.0		864.8
VEBAs
Segment operating (loss) income:
Non-cash net periodic postretirement benefit charges (income) relating to VEBAs1	0.6	(5.6)
Pre-tax loss on settlement of Union VEBA	$ 492.2	$ 0

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