Supplementary Guarantor Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	$ 1,658.9	$ 1,533.4
Cost of products sold (exclusive of item shown separately below)	1,463.7	1,276.9
Selling and administrative expenses (exclusive of item shown separately below)	76.7	71.7
Depreciation	54.9	55.1
Total operating costs	1,595.3	1,403.7
Operating profit (loss)	63.6	129.7
Interest expense	37.6	39.4
Pension and OPEB (income) expense	(10.0)	(18.1)
Other (income) expense	(3.9)	7.8
Income (loss) before income taxes	39.9	100.6
Income tax expense (benefit)	(4.9)	0.0
Equity in net income (loss) of subsidiaries	0.0	0.0
Net income	44.8	100.6
Less: Net income attributable to noncontrolling interests	16.1	16.2
Net income attributable to AK Steel Holding Corporation	28.7	84.4
Other comprehensive income (loss)	(6.7)	(20.1)
Comprehensive income attributable to AK Steel Holding Corporation	22.0	64.3
Current assets:
Cash and cash equivalents	44.5	185.7	$ 44.5	$ 38.0	$ 185.7	$ 173.2
Accounts receivable, net			619.8	517.8
Inventory			1,334.0	1,385.0
Other current assets			96.4	130.3
Total current assets			2,094.7	2,071.1
Property, plant and equipment			6,850.7	6,831.8
Accumulated depreciation			(4,899.7)	(4,845.6)
Property, plant and equipment, net			1,951.0	1,986.2
Investment in subsidiaries			0.0	0.0
Inter-company accounts			0.0	0.0
Other non-current assets			412.0	417.5
TOTAL ASSETS			4,457.7	4,474.8
Current liabilities:
Accounts payable			728.6	690.4
Accrued liabilities			234.1	270.5
Current portion of pension and other postretirement benefit obligations			39.7	40.1
Total current liabilities			1,002.4	1,001.0
Non-current liabilities:
Long-term debt			2,103.3	2,110.1
Pension and other postretirement benefit obligations			869.2	894.2
Inter-company accounts			0.0	0.0
Other non-current liabilities			150.9	168.9
TOTAL LIABILITIES			4,125.8	4,174.2
Equity [Abstract]
Total stockholders' equity (deficit)			(19.1)	(44.6)
Noncontrolling interests			351.0	345.2
TOTAL EQUITY			331.9	300.6	218.4	149.8
TOTAL LIABILITIES AND EQUITY			4,457.7	4,474.8
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities	65.6	36.3
Cash flows from investing activities:
Capital investments	(37.9)	(32.5)
Other investing items, net	0.0	1.5
Net cash flows from investing activities	(37.9)	(31.0)
Cash flows from financing activities:
Net borrowings (payments) under credit facility	(10.0)	0.0
Proceeds from issuance of long-term debt	0.0	400.0
Redemption of long-term debt	0.0	(367.5)
Debt issuance costs	0.0	(7.8)
Inter-company activity	0.0	0.0
SunCoke Middletown distributions to noncontrolling interest owners	(10.3)	(15.1)
Other financing items, net	(0.9)	(2.4)
Net cash flows from financing activities	(21.2)	7.2
Net increase (decrease) in cash and cash equivalents	6.5	12.5
Cash and cash equivalents, beginning of period	38.0	173.2
Cash and cash equivalents, end of period	44.5	185.7
AK Holding [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	0.0	0.0
Cost of products sold (exclusive of item shown separately below)	0.0	0.0
Selling and administrative expenses (exclusive of item shown separately below)	1.3	1.4
Depreciation	0.0	0.0
Total operating costs	1.3	1.4
Operating profit (loss)	(1.3)	(1.4)
Interest expense	0.0	0.0
Pension and OPEB (income) expense	0.0	0.0
Other (income) expense	0.0	0.0
Income (loss) before income taxes	(1.3)	(1.4)
Income tax expense (benefit)	0.0	0.0
Equity in net income (loss) of subsidiaries	30.0	85.8
Net income	28.7	84.4
Less: Net income attributable to noncontrolling interests	0.0	0.0
Net income attributable to AK Steel Holding Corporation	28.7	84.4
Other comprehensive income (loss)	(6.7)	(20.1)
Comprehensive income attributable to AK Steel Holding Corporation	22.0	64.3
Current assets:
Cash and cash equivalents	0.0	0.0	0.0	0.0	0.0	0.0
Accounts receivable, net			0.0	0.0
Inventory			0.0	0.0
Other current assets			0.0	0.0
Total current assets			0.0	0.0
Property, plant and equipment			0.0	0.0
Accumulated depreciation			0.0	0.0
Property, plant and equipment, net			0.0	0.0
Investment in subsidiaries			(3,170.1)	(3,127.4)
Inter-company accounts			3,151.0	3,082.8
Other non-current assets			0.0	0.0
TOTAL ASSETS			(19.1)	(44.6)
Current liabilities:
Accounts payable			0.0	0.0
Accrued liabilities			0.0	0.0
Current portion of pension and other postretirement benefit obligations			0.0	0.0
Total current liabilities			0.0	0.0
Non-current liabilities:
Long-term debt			0.0	0.0
Pension and other postretirement benefit obligations			0.0	0.0
Inter-company accounts			0.0	0.0
Other non-current liabilities			0.0	0.0
TOTAL LIABILITIES			0.0	0.0
Equity [Abstract]
Total stockholders' equity (deficit)			(19.1)	(44.6)
Noncontrolling interests			0.0	0.0
TOTAL EQUITY			(19.1)	(44.6)
TOTAL LIABILITIES AND EQUITY			(19.1)	(44.6)
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities	(1.0)	(1.1)
Cash flows from investing activities:
Capital investments	0.0	0.0
Other investing items, net	0.0	0.0
Net cash flows from investing activities	0.0	0.0
Cash flows from financing activities:
Net borrowings (payments) under credit facility	0.0
Proceeds from issuance of long-term debt		0.0
Redemption of long-term debt		0.0
Debt issuance costs		0.0
Inter-company activity	2.0	3.6
SunCoke Middletown distributions to noncontrolling interest owners	0.0	0.0
Other financing items, net	(1.0)	(2.5)
Net cash flows from financing activities	1.0	1.1
Net increase (decrease) in cash and cash equivalents	0.0	0.0
Cash and cash equivalents, beginning of period	0.0	0.0
Cash and cash equivalents, end of period	0.0	0.0
AK Steel [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	1,503.0	1,473.6
Cost of products sold (exclusive of item shown separately below)	1,353.4	1,258.0
Selling and administrative expenses (exclusive of item shown separately below)	74.9	72.8
Depreciation	43.4	47.2
Total operating costs	1,471.7	1,378.0
Operating profit (loss)	31.3	95.6
Interest expense	36.4	39.0
Pension and OPEB (income) expense	(10.0)	(18.1)
Other (income) expense	1.2	11.4
Income (loss) before income taxes	3.7	63.3
Income tax expense (benefit)	(11.9)	(8.6)
Equity in net income (loss) of subsidiaries	14.4	13.9
Net income	30.0	85.8
Less: Net income attributable to noncontrolling interests	0.0	0.0
Net income attributable to AK Steel Holding Corporation	30.0	85.8
Other comprehensive income (loss)	(6.7)	(20.1)
Comprehensive income attributable to AK Steel Holding Corporation	23.3	65.7
Current assets:
Cash and cash equivalents	14.5	147.9	12.7	14.5	161.1	147.9
Accounts receivable, net			499.0	432.4
Inventory			1,202.3	1,231.5
Other current assets			89.1	124.7
Total current assets			1,803.1	1,803.1
Property, plant and equipment			6,019.5	6,004.0
Accumulated depreciation			(4,663.8)	(4,620.8)
Property, plant and equipment, net			1,355.7	1,383.2
Investment in subsidiaries			1,886.9	1,844.9
Inter-company accounts			0.0	0.0
Other non-current assets			60.7	66.3
TOTAL ASSETS			5,106.4	5,097.5
Current liabilities:
Accounts payable			661.7	623.3
Accrued liabilities			195.3	228.2
Current portion of pension and other postretirement benefit obligations			39.3	39.7
Total current liabilities			896.3	891.2
Non-current liabilities:
Long-term debt			2,103.3	2,110.1
Pension and other postretirement benefit obligations			865.7	890.8
Inter-company accounts			4,294.5	4,199.6
Other non-current liabilities			116.7	133.2
TOTAL LIABILITIES			8,276.5	8,224.9
Equity [Abstract]
Total stockholders' equity (deficit)			(3,170.1)	(3,127.4)
Noncontrolling interests			0.0	0.0
TOTAL EQUITY			(3,170.1)	(3,127.4)
TOTAL LIABILITIES AND EQUITY			5,106.4	5,097.5
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities	45.7	16.0
Cash flows from investing activities:
Capital investments	(31.4)	(30.6)
Other investing items, net	0.0	1.4
Net cash flows from investing activities	(31.4)	(29.2)
Cash flows from financing activities:
Net borrowings (payments) under credit facility	(10.0)
Proceeds from issuance of long-term debt		400.0
Redemption of long-term debt		(367.5)
Debt issuance costs		(7.8)
Inter-company activity	(6.2)	1.6
SunCoke Middletown distributions to noncontrolling interest owners	0.0	0.0
Other financing items, net	0.1	0.1
Net cash flows from financing activities	(16.1)	26.4
Net increase (decrease) in cash and cash equivalents	(1.8)	13.2
Cash and cash equivalents, beginning of period	14.5	147.9
Cash and cash equivalents, end of period	12.7	161.1
Guarantor Subsidiaries [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	77.1	72.3
Cost of products sold (exclusive of item shown separately below)	54.6	46.5
Selling and administrative expenses (exclusive of item shown separately below)	3.6	3.2
Depreciation	2.0	2.0
Total operating costs	60.2	51.7
Operating profit (loss)	16.9	20.6
Interest expense	0.0	0.0
Pension and OPEB (income) expense		0.0
Other (income) expense	(3.5)	(2.4)
Income (loss) before income taxes	20.4	23.0
Income tax expense (benefit)	5.1	8.8
Equity in net income (loss) of subsidiaries	0.0	0.0
Net income	15.3	14.2
Less: Net income attributable to noncontrolling interests	0.0	0.0
Net income attributable to AK Steel Holding Corporation	15.3	14.2
Other comprehensive income (loss)	0.0	0.0
Comprehensive income attributable to AK Steel Holding Corporation	15.3	14.2
Current assets:
Cash and cash equivalents	7.2	4.4	4.1	7.2	2.6	4.4
Accounts receivable, net			38.6	33.2
Inventory			57.6	55.1
Other current assets			0.1	0.2
Total current assets			100.4	95.7
Property, plant and equipment			182.3	181.5
Accumulated depreciation			(96.8)	(94.8)
Property, plant and equipment, net			85.5	86.7
Investment in subsidiaries			0.0	0.0
Inter-company accounts			1,565.7	1,523.1
Other non-current assets			33.1	33.0
TOTAL ASSETS			1,784.7	1,738.5
Current liabilities:
Accounts payable			21.0	17.9
Accrued liabilities			6.7	7.8
Current portion of pension and other postretirement benefit obligations			0.0	0.0
Total current liabilities			27.7	25.7
Non-current liabilities:
Long-term debt			0.0	0.0
Pension and other postretirement benefit obligations			0.0	0.0
Inter-company accounts			0.0	0.0
Other non-current liabilities			0.0	1.0
TOTAL LIABILITIES			27.7	26.7
Equity [Abstract]
Total stockholders' equity (deficit)			1,757.0	1,711.8
Noncontrolling interests			0.0	0.0
TOTAL EQUITY			1,757.0	1,711.8
TOTAL LIABILITIES AND EQUITY			1,784.7	1,738.5
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities	4.8	1.1
Cash flows from investing activities:
Capital investments	(0.9)	(1.4)
Other investing items, net	0.0	0.0
Net cash flows from investing activities	(0.9)	(1.4)
Cash flows from financing activities:
Net borrowings (payments) under credit facility	0.0
Proceeds from issuance of long-term debt		0.0
Redemption of long-term debt		0.0
Debt issuance costs		0.0
Inter-company activity	(7.0)	(1.5)
SunCoke Middletown distributions to noncontrolling interest owners	0.0	0.0
Other financing items, net	0.0	0.0
Net cash flows from financing activities	(7.0)	(1.5)
Net increase (decrease) in cash and cash equivalents	(3.1)	(1.8)
Cash and cash equivalents, beginning of period	7.2	4.4
Cash and cash equivalents, end of period	4.1	2.6
Other Non-Guarantor Subsidiaries [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	208.7	91.4
Cost of products sold (exclusive of item shown separately below)	171.7	62.8
Selling and administrative expenses (exclusive of item shown separately below)	8.7	5.5
Depreciation	9.5	5.9
Total operating costs	189.9	74.2
Operating profit (loss)	18.8	17.2
Interest expense	1.2	0.4
Pension and OPEB (income) expense		0.0
Other (income) expense	(1.6)	(1.2)
Income (loss) before income taxes	19.2	18.0
Income tax expense (benefit)	2.4	0.7
Equity in net income (loss) of subsidiaries	0.0	0.1
Net income	16.8	17.4
Less: Net income attributable to noncontrolling interests	16.1	16.2
Net income attributable to AK Steel Holding Corporation	0.7	1.2
Other comprehensive income (loss)	1.1	0.3
Comprehensive income attributable to AK Steel Holding Corporation	1.8	1.5
Current assets:
Cash and cash equivalents	16.3	20.9	27.7	16.3	22.0	20.9
Accounts receivable, net			103.4	65.4
Inventory			86.1	108.3
Other current assets			7.2	5.4
Total current assets			224.4	195.4
Property, plant and equipment			648.9	646.3
Accumulated depreciation			(139.1)	(130.0)
Property, plant and equipment, net			509.8	516.3
Investment in subsidiaries			67.5	67.5
Inter-company accounts			0.0	0.0
Other non-current assets			318.2	318.2
TOTAL ASSETS			1,119.9	1,097.4
Current liabilities:
Accounts payable			50.6	50.9
Accrued liabilities			32.1	34.5
Current portion of pension and other postretirement benefit obligations			0.4	0.4
Total current liabilities			83.1	85.8
Non-current liabilities:
Long-term debt			0.0	0.0
Pension and other postretirement benefit obligations			3.5	3.4
Inter-company accounts			510.8	489.1
Other non-current liabilities			34.2	34.7
TOTAL LIABILITIES			631.6	613.0
Equity [Abstract]
Total stockholders' equity (deficit)			137.3	139.2
Noncontrolling interests			351.0	345.2
TOTAL EQUITY			488.3	484.4
TOTAL LIABILITIES AND EQUITY			1,119.9	1,097.4
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities	11.1	21.4
Cash flows from investing activities:
Capital investments	(5.6)	(0.5)
Other investing items, net	0.0	0.1
Net cash flows from investing activities	(5.6)	(0.4)
Cash flows from financing activities:
Net borrowings (payments) under credit facility	0.0
Proceeds from issuance of long-term debt		0.0
Redemption of long-term debt		0.0
Debt issuance costs		0.0
Inter-company activity	16.2	(4.8)
SunCoke Middletown distributions to noncontrolling interest owners	(10.3)	(15.1)
Other financing items, net	0.0	0.0
Net cash flows from financing activities	5.9	(19.9)
Net increase (decrease) in cash and cash equivalents	11.4	1.1
Cash and cash equivalents, beginning of period	16.3	20.9
Cash and cash equivalents, end of period	27.7	22.0
Consolidation, Eliminations [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	(129.9)	(103.9)
Cost of products sold (exclusive of item shown separately below)	(116.0)	(90.4)
Selling and administrative expenses (exclusive of item shown separately below)	(11.8)	(11.2)
Depreciation	0.0	0.0
Total operating costs	(127.8)	(101.6)
Operating profit (loss)	(2.1)	(2.3)
Interest expense	0.0	0.0
Pension and OPEB (income) expense		0.0
Other (income) expense	0.0	0.0
Income (loss) before income taxes	(2.1)	(2.3)
Income tax expense (benefit)	(0.5)	(0.9)
Equity in net income (loss) of subsidiaries	(44.4)	(99.8)
Net income	(46.0)	(101.2)
Less: Net income attributable to noncontrolling interests	0.0	0.0
Net income attributable to AK Steel Holding Corporation	(46.0)	(101.2)
Other comprehensive income (loss)	5.6	19.8
Comprehensive income attributable to AK Steel Holding Corporation	(40.4)	(81.4)
Current assets:
Cash and cash equivalents	0.0	0.0	0.0	0.0	$ 0.0	$ 0.0
Accounts receivable, net			(21.2)	(13.2)
Inventory			(12.0)	(9.9)
Other current assets			0.0	0.0
Total current assets			(33.2)	(23.1)
Property, plant and equipment			0.0	0.0
Accumulated depreciation			0.0	0.0
Property, plant and equipment, net			0.0	0.0
Investment in subsidiaries			1,215.7	1,215.0
Inter-company accounts			(4,716.7)	(4,605.9)
Other non-current assets			0.0	0.0
TOTAL ASSETS			(3,534.2)	(3,414.0)
Current liabilities:
Accounts payable			(4.7)	(1.7)
Accrued liabilities			0.0	0.0
Current portion of pension and other postretirement benefit obligations			0.0	0.0
Total current liabilities			(4.7)	(1.7)
Non-current liabilities:
Long-term debt			0.0	0.0
Pension and other postretirement benefit obligations			0.0	0.0
Inter-company accounts			(4,805.3)	(4,688.7)
Other non-current liabilities			0.0	0.0
TOTAL LIABILITIES			(4,810.0)	(4,690.4)
Equity [Abstract]
Total stockholders' equity (deficit)			1,275.8	1,276.4
Noncontrolling interests			0.0	0.0
TOTAL EQUITY			1,275.8	1,276.4
TOTAL LIABILITIES AND EQUITY			$ (3,534.2)	$ (3,414.0)
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities	5.0	(1.1)
Cash flows from investing activities:
Capital investments	0.0	0.0
Other investing items, net	0.0	0.0
Net cash flows from investing activities	0.0	0.0
Cash flows from financing activities:
Net borrowings (payments) under credit facility	0.0
Proceeds from issuance of long-term debt		0.0
Redemption of long-term debt		0.0
Debt issuance costs		0.0
Inter-company activity	(5.0)	1.1
SunCoke Middletown distributions to noncontrolling interest owners	0.0	0.0
Other financing items, net	0.0	0.0
Net cash flows from financing activities	(5.0)	1.1
Net increase (decrease) in cash and cash equivalents	0.0	0.0
Cash and cash equivalents, beginning of period	0.0	0.0
Cash and cash equivalents, end of period	$ 0.0	$ 0.0