Summary of Significant Accounting Policies Accounting Change (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Accounting Changes [Abstract]
Pension and OPEB income	$ 10.0	$ 18.1

Indicates line item in statement of income or comprehensive income that includes components of net periodic benefit cost (credit), excluding service cost component, for defined benefit plan.

No definition available.

Name:	aks_DefinedBenefitPlanNetPeriodicBenefitCostCreditExcludingServiceCost
Namespace Prefix:	aks_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration