Long-term Debt and Other Financing (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2017	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Unamortized debt discount and issuance costs		$ (52.2)	$ (55.4)
Total long-term debt		2,103.3	2,110.1
Gain (Loss) on Repurchase of Debt Instrument	$ (9.7)
Senior Secured Notes Due July 2023 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 380.0	$ 380.0
Debt Instrument, Interest Rate, Stated Percentage		7.50%	7.50%
Debt Instrument, Interest Rate, Effective Percentage		8.30%	8.30%
Exchangeable Senior Notes Due November 2019 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 150.0	$ 150.0
Debt Instrument, Interest Rate, Stated Percentage		5.00%	5.00%
Debt Instrument, Interest Rate, Effective Percentage		10.80%	10.80%
Senior Notes Due October 2021 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 406.2	$ 406.2
Debt Instrument, Interest Rate, Stated Percentage		7.625%	7.625%
Senior Notes Due October 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 280.0	$ 280.0
Debt Instrument, Interest Rate, Stated Percentage		6.375%	6.375%
Debt Instrument, Interest Rate, Effective Percentage		7.10%	7.10%
Senior Notes Due March 2027 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 400.0	$ 400.0
Debt Instrument, Interest Rate, Stated Percentage		7.00%	7.00%
Industrial Revenue Bonds [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 99.3	$ 99.3

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateEffectivePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of unamortized debt discount (premium) and debt issuance costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

+ Details

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

- Definition

Amount of gain (loss) from the difference between the repurchase price of a debt instrument initially issued by the entity and the net carrying amount of the debt at the time of its repurchase.

+ References

No definition available.

+ Details

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

- Definition

Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

+ Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=aks_SeniorSecuredNotesDueJuly2023Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=aks_ExchangeableSeniorNotesDueNovember2019Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=aks_SeniorNotesDueOctober2021Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=aks_SeniorNotesDueOctober2025Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=aks_SeniorNotesDueMarch2027Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=aks_IndustrialRevenueBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: