Number Of Major Customers

No definition available.

Name:	aks_NumberOfMajorCustomers
Namespace Prefix:	aks_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of persons employed by the Entity

+ References

No definition available.

+ Details

Name:	dei_EntityNumberOfEmployees
Namespace Prefix:	dei_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

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

- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13531-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13537-108611

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

+ Details

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Period the derivative contract is outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeTermOfContract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13854-109267

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13777-109266

+ Details

Name:	us-gaap_GoodwillImpairmentLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of expense (reversal of expense) for net periodic benefit cost components, excluding service cost component, of defined benefit plan. Amount includes, but is not limited to, interest cost, expected (return) loss on plan asset, amortization of prior service cost (credit), amortization of (gain) loss, amortization of transition (asset) obligation, settlement (gain) loss, curtailment (gain) loss and certain termination benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 45
-Paragraph 3A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109237824&loc=SL108413299-114919

+ Details

Name:	us-gaap_NetPeriodicDefinedBenefitsExpenseReversalOfExpenseExcludingServiceCostComponent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Present value of lessee's discounted obligation for lease payments from operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977

+ Details

Name:	us-gaap_OperatingLeaseLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of lessee's right to use underlying asset under operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977

+ Details

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

- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

Name:	us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Summary of Significant Accounting Policies (Details2)	12 Months Ended
Summary of Significant Accounting Policies (Details2)	Dec. 31, 2018 USD ($) employees	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2019 USD ($)
Concentration Of Risk [Line Items]
Number of Employees	9,500
Pension and OPEB (income) expense \| $	$ (19,200,000)	$ (71,900,000)	$ 16,500,000
Goodwill, Impairment Loss \| $	$ 0	$ 0	$ 0
Workforce Subject to Collective Bargaining Arrangements [Member]
Concentration Of Risk [Line Items]
Number of Employees	5,700
Workforce Subject to Collective Bargaining Arrangements Expiring within One Year [Member] \| Mountain State Carbon [Member]
Concentration Of Risk [Line Items]
Number of Employees	230
Workforce Subject to Collective Bargaining Arrangements Expiring within One Year [Member] \| Coshocton Works [Member]
Concentration Of Risk [Line Items]
Number of Employees	300
Customer Concentration Risk [Member] \| Sales Revenue, Goods, Net [Member]
Concentration Of Risk [Line Items]
Number Of Major Customers	2	1	2
Customer Concentration Risk [Member] \| Major Customer A [Member] \| Sales Revenue, Goods, Net [Member]
Concentration Of Risk [Line Items]
Percentage of total (in hundredths)	11.00%	12.00%	12.00%
Customer Concentration Risk [Member] \| Major Customer B [Member] \| Sales Revenue, Goods, Net [Member]
Concentration Of Risk [Line Items]
Percentage of total (in hundredths)	10.00%		11.00%
Credit Concentration Risk [Member] \| Accounts Receivable [Member]
Concentration Of Risk [Line Items]
Number Of Major Customers	1	1
Credit Concentration Risk [Member] \| Automotive Market [Member] \| Accounts Receivable [Member]
Concentration Of Risk [Line Items]
Percentage of total (in hundredths)	52.00%	55.00%
Credit Concentration Risk [Member] \| Major Customer A [Member] \| Accounts Receivable [Member]
Concentration Of Risk [Line Items]
Percentage of total (in hundredths)	13.00%	18.00%
Maximum [Member] \| Commodity [Member]
Concentration Of Risk [Line Items]
Derivative contracts duration (in months)	2 years
Maximum [Member] \| Forward Contracts [Member]
Concentration Of Risk [Line Items]
Derivative contracts duration (in months)	3 years
IAM National Pension Fund's National Pension Plan [Member] \| Workforce Subject to Collective Bargaining Arrangements [Member] \| Middletown Works [Member]
Concentration Of Risk [Line Items]
Number of Employees	1,760
IAM National Pension Fund's National Pension Plan [Member] \| Workforce Subject to Collective Bargaining Arrangements Expiring within One Year [Member] \| Butler Works [Member]
Concentration Of Risk [Line Items]
Number of Employees	1,170
IAM National Pension Fund's National Pension Plan [Member] \| Workforce Subject to Collective Bargaining Arrangements Expiring within One Year [Member] \| Zanesville Works [Member]
Concentration Of Risk [Line Items]
Number of Employees	100
Machinery and equipment [Member] \| Maximum [Member]
Concentration Of Risk [Line Items]
Property, Plant and Equipment, Useful Life	20 years
Machinery and equipment [Member] \| Minimum [Member]
Concentration Of Risk [Line Items]
Property, Plant and Equipment, Useful Life	2 years
Accounting Standards Update 2016-02 [Member] \| Scenario, Forecast [Member] \| Maximum [Member]
Concentration Of Risk [Line Items]
Operating Lease, Right-of-use Asset \| $				$ 350,000,000.0
Operating Lease, Liability \| $				350,000,000
Accounting Standards Update 2016-02 [Member] \| Scenario, Forecast [Member] \| Minimum [Member]
Concentration Of Risk [Line Items]
Operating Lease, Right-of-use Asset \| $				250,000,000.0
Operating Lease, Liability \| $				$ 250,000,000