Summary of Significant Accounting Policies (Policies)	3 Months Ended
Mar. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Accounting, Policy	Basis of Presentation: These financial statements consolidate the operations and accounts of AK Steel Holding Corporation (“AK Holding”), its wholly-owned subsidiary AK Steel Corporation (“AK Steel”), all subsidiaries in which AK Holding has a controlling interest, and two variable interest entities for which AK Steel is the primary beneficiary. Unless the context provides otherwise, references to “we,” “us” and “our” refer to AK Holding and its subsidiaries. In our opinion, the accompanying condensed consolidated financial statements contain all adjustments, consisting of normal recurring adjustments, necessary to present fairly our financial position as of March 31, 2018 and December 31, 2017, our results of operations for the three months ended March 31, 2018 and 2017, and our cash flows for the three months ended March 31, 2018 and 2017. Our results of operations for the three months ended March 31, 2018 are not necessarily indicative of the results we expect for the full year ending December 31, 2018. These condensed consolidated financial statements should be read along with our audited consolidated financial statements for the year ended December 31, 2017, included in our Annual Report on Form 10-K for the year ended December 31, 2017.
Revenue Recognition, Policy [Policy Text Block]	Revenue Recognition: We generate our revenue through product sales, and shipping terms generally indicate when we have fulfilled our performance obligations and passed control of products to our customer. Our revenue transactions consist of a single performance obligation to transfer promised goods. We have contracts with a substantial portion of our customers. These contracts usually define the mechanism for determining the sales price, but the contacts do not impose a specific quantity on either party. Quantities to be delivered to the customer are determined at a point near the date of delivery through purchase orders or other written instructions we receive from the customer. Spot market sales are made through purchase orders or other written instructions. We recognize revenue when we have fulfilled a performance obligation, which is typically when we have shipped products at the customer’s instructions. For sales with shipping terms that transfer title and risk of loss at the destination point, we recognize revenue when the customer receives the goods and our performance obligation is complete. For sales with shipping terms that transfer title and risk of loss at the shipping point with us bearing responsibility for freight costs to the destination, we determine that we have fulfilled a single performance obligation and recognize revenue when we ship the goods. For our tooling solutions, we record progress payments that we receive from a customer as accrued liabilities until we recognize the revenue when the customer provides written acceptance that our performance obligation has been fulfilled. Revenue is measured as the amount of consideration we expect to receive in exchange for transferring product. We reduce the amount of revenue recognized for estimated returns and other customer credits, such as discounts and volume rebates, based on the expected value to be realized. Payment terms are consistent with terms standard to the markets we serve. We maintain an allowance for doubtful accounts for the loss that would be incurred if a customer is unable to pay amounts due. We initially estimate the allowance required at the time of revenue recognition based on historical experience and make changes to the allowance based on various factors, including changes in the customer’s financial condition or payment patterns. Sales taxes collected from customers are excluded from revenues.
Inventory, Policy [Policy Text Block]	Inventory: Inventories are valued at the lower of cost or net realizable value. We measure the cost of inventories using the average cost method. See Note 3 for information on the change to the average cost method effective January 1, 2018.
Accounting Changes [Text Block]	Pension and Other Postretirement Benefits: On January 1, 2018, we adopted Accounting Standards Update No. 2017-07, Compensation—Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. Under this standard, the service cost component of periodic pension and other postretirement benefit expense is included in cost of products sold and selling and administrative expenses, consistent with our treatment of other employee compensation costs. The remainder of periodic pension and other postretirement benefit income of $10.0 and $18.1 for the three months ended March 31, 2018 and 2017, is recorded in a separate line in the condensed consolidated statements of operations below operating profit. We have retrospectively applied the change in accounting principle to all periods presented. The adoption of this standard update had no other effect on our financial statements.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Basis of Presentation: These financial statements consolidate the operations and accounts of AK Steel Holding Corporation (“AK Holding”), its wholly-owned subsidiary AK Steel Corporation (“AK Steel”), all subsidiaries in which AK Holding has a controlling interest, and two variable interest entities for which AK Steel is the primary beneficiary. Unless the context provides otherwise, references to “we,” “us” and “our” refer to AK Holding and its subsidiaries. In our opinion, the accompanying condensed consolidated financial statements contain all adjustments, consisting of normal recurring adjustments, necessary to present fairly our financial position as of March 31, 2018 and December 31, 2017, our results of operations for the three months ended March 31, 2018 and 2017, and our cash flows for the three months ended March 31, 2018 and 2017. Our results of operations for the three months ended March 31, 2018 are not necessarily indicative of the results we expect for the full year ending December 31, 2018. These condensed consolidated financial statements should be read along with our audited consolidated financial statements for the year ended December 31, 2017, included in our Annual Report on Form 10-K for the year ended December 31, 2017.

Revenue Recognition, Policy [Policy Text Block]

Revenue Recognition: We generate our revenue through product sales, and shipping terms generally indicate when we have fulfilled our performance obligations and passed control of products to our customer. Our revenue transactions consist of a single performance obligation to transfer promised goods. We have contracts with a substantial portion of our customers. These contracts usually define the mechanism for determining the sales price, but the contacts do not impose a specific quantity on either party. Quantities to be delivered to the customer are determined at a point near the date of delivery through purchase orders or other written instructions we receive from the customer. Spot market sales are made through purchase orders or other written instructions. We recognize revenue when we have fulfilled a performance obligation, which is typically when we have shipped products at the customer’s instructions. For sales with shipping terms that transfer title and risk of loss at the destination point, we recognize revenue when the customer receives the goods and our performance obligation is complete. For sales with shipping terms that transfer title and risk of loss at the shipping point with us bearing responsibility for freight costs to the destination, we determine that we have fulfilled a single performance obligation and recognize revenue when we ship the goods. For our tooling solutions, we record progress payments that we receive from a customer as accrued liabilities until we recognize the revenue when the customer provides written acceptance that our performance obligation has been fulfilled.

Revenue is measured as the amount of consideration we expect to receive in exchange for transferring product. We reduce the amount of revenue recognized for estimated returns and other customer credits, such as discounts and volume rebates, based on the expected value to be realized. Payment terms are consistent with terms standard to the markets we serve. We maintain an allowance for doubtful accounts for the loss that would be incurred if a customer is unable to pay amounts due. We initially estimate the allowance required at the time of revenue recognition based on historical experience and make changes to the allowance based on various factors, including changes in the customer’s financial condition or payment patterns. Sales taxes collected from customers are excluded from revenues.

Inventory, Policy [Policy Text Block]

Inventory: Inventories are valued at the lower of cost or net realizable value. We measure the cost of inventories using the average cost method. See Note 3 for information on the change to the average cost method effective January 1, 2018.

Accounting Changes [Text Block]

Pension and Other Postretirement Benefits: On January 1, 2018, we adopted Accounting Standards Update No. 2017-07, Compensation—Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. Under this standard, the service cost component of periodic pension and other postretirement benefit expense is included in cost of products sold and selling and administrative expenses, consistent with our treatment of other employee compensation costs. The remainder of periodic pension and other postretirement benefit income of $10.0 and $18.1 for the three months ended March 31, 2018 and 2017, is recorded in a separate line in the condensed consolidated statements of operations below operating profit. We have retrospectively applied the change in accounting principle to all periods presented. The adoption of this standard update had no other effect on our financial statements.