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Acquisition of Dearborn (Details) (Severstal Dearborn [Member], USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Sep. 16, 2014
Severstal Dearborn [Member]
     
Business Acquisition [Line Items]      
Accounts receivable     $ 154.4us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables
/ us-gaap_BusinessAcquisitionAxis
= aks_SeverstalDearbornMember
Inventory     362.2us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventory
/ us-gaap_BusinessAcquisitionAxis
= aks_SeverstalDearbornMember
Other current assets     3.6us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsOther
/ us-gaap_BusinessAcquisitionAxis
= aks_SeverstalDearbornMember
Property, plant and equipment     459.3us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment
/ us-gaap_BusinessAcquisitionAxis
= aks_SeverstalDearbornMember
Investment in affilates     88.2aks_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedEquityMethodInvestments
/ us-gaap_BusinessAcquisitionAxis
= aks_SeverstalDearbornMember
Total assets acquired     1,067.7us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets
/ us-gaap_BusinessAcquisitionAxis
= aks_SeverstalDearbornMember
Accounts payable     (201.2)us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable
/ us-gaap_BusinessAcquisitionAxis
= aks_SeverstalDearbornMember
Accrued liabilities     (36.1)us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther
/ us-gaap_BusinessAcquisitionAxis
= aks_SeverstalDearbornMember
Other postretirement benefit obligations     (128.2)aks_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherPostretirementBenefits
/ us-gaap_BusinessAcquisitionAxis
= aks_SeverstalDearbornMember
Other non-current liabilities     (11.9)us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther
/ us-gaap_BusinessAcquisitionAxis
= aks_SeverstalDearbornMember
Total liabilities assumed     (377.4)us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities
/ us-gaap_BusinessAcquisitionAxis
= aks_SeverstalDearbornMember
Purchase price, net of cash acquired     690.3us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet
/ us-gaap_BusinessAcquisitionAxis
= aks_SeverstalDearbornMember
Business Acquisition, Pro Forma Revenue 7,942.7us-gaap_BusinessAcquisitionsProFormaRevenue
/ us-gaap_BusinessAcquisitionAxis
= aks_SeverstalDearbornMember
7,601.6us-gaap_BusinessAcquisitionsProFormaRevenue
/ us-gaap_BusinessAcquisitionAxis
= aks_SeverstalDearbornMember
 
Business Acquisition, Pro Forma Operating Profit (Loss) $ (816.2)aks_BusinessAcquisitionProFormaOperatingProfitLoss
/ us-gaap_BusinessAcquisitionAxis
= aks_SeverstalDearbornMember
$ 170.5aks_BusinessAcquisitionProFormaOperatingProfitLoss
/ us-gaap_BusinessAcquisitionAxis
= aks_SeverstalDearbornMember