When an investment contract or contracts have a range of settlement dates, the earliest settlement date.

No definition available.

Name:	aks_InvestmentContractSettlementDateRangeBegin
Namespace Prefix:	aks_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

When an investment contract or contracts have a range of settlement dates, the latest settlement date.

+ References

No definition available.

+ Details

Name:	aks_InvestmentContractSettlementDateRangeEnding
Namespace Prefix:	aks_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of right to receive cash collateral under master netting arrangements that have not been offset against derivative liabilities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226016-175313

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=d3e41678-113959

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

Name:	us-gaap_DerivativeCollateralRightToReclaimCash
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The estimated value of gains (losses), net anticipated to be transferred in the future from accumulated other comprehensive income into earnings.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=112277219&loc=d3e80845-113994

+ Details

Name:	us-gaap_DerivativeInstrumentsGainLossReclassificationFromAccumulatedOCIToIncomeEstimatedNetAmountToBeTransferred
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Nominal number of units used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNonmonetaryNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_CommodityContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	aks_DerivativeCommodityContractByCommodityAxis=aks_NaturalGasMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	aks_DerivativeCommodityContractByCommodityAxis=aks_ZincMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	aks_DerivativeCommodityContractByCommodityAxis=aks_IronOreMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	aks_DerivativeCommodityContractByCommodityAxis=us-gaap_ElectricityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CurrencyAxis=currency_EUR
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CurrencyAxis=currency_CAD
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Derivative Instruments and Hedging Activities (Details) € in Millions, $ in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($) MMBTU MW t lb	Sep. 30, 2019 CAD ($) MMBTU MW t lb	Sep. 30, 2019 EUR (€) MMBTU MW t lb	Dec. 31, 2018 CAD ($) MMBTU MW t lb	Dec. 31, 2018 EUR (€) MMBTU MW t lb
Derivative [Line Items]
Derivative, Collateral, Right to Reclaim Cash	$ 0.0
Commodity Contract [Member] \| Natural Gas [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount \| MMBTU	37,905,000	37,905,000	37,905,000	39,868,000	39,868,000
Beginning Settlement Date	Oct. 01, 2019
Ending Settlement Date	Jun. 30, 2021
Gains (losses) expected to be reclassified into cost of products sold within the next twelve months	$ (9.9)
Commodity Contract [Member] \| Zinc [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount \| lb	33,050,000	33,050,000	33,050,000	52,150,000	52,150,000
Beginning Settlement Date	Oct. 01, 2019
Ending Settlement Date	Jun. 30, 2021
Gains (losses) expected to be reclassified into cost of products sold within the next twelve months	$ (3.8)
Commodity Contract [Member] \| Iron Ore [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount \| t	1,810,000	1,810,000	1,810,000	2,125,000	2,125,000
Beginning Settlement Date	Oct. 01, 2019
Ending Settlement Date	Jun. 30, 2021
Commodity Contract [Member] \| Electricity [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount \| MW	1,597,000	1,597,000	1,597,000	1,461,000	1,461,000
Beginning Settlement Date	Oct. 01, 2019
Ending Settlement Date	Jun. 30, 2021
Gains (losses) expected to be reclassified into cost of products sold within the next twelve months	$ (1.7)
Foreign Exchange Contract [Member] \| Euro Member Countries, Euro
Derivative [Line Items]
Derivative, Notional Amount \| €			€ 1.0		€ 4.0
Beginning Settlement Date	Oct. 01, 2019
Ending Settlement Date	Nov. 30, 2019
Foreign Exchange Contract [Member] \| Canada, Dollars
Derivative [Line Items]
Derivative, Notional Amount		$ 91.1		$ 118.6
Beginning Settlement Date	Oct. 01, 2019
Ending Settlement Date	Dec. 31, 2021
Gains (losses) expected to be reclassified into cost of products sold within the next twelve months	$ (1.5)