Supplementary Guarantor Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	$ 1,535.5	$ 1,735.6	$ 4,913.7	$ 5,141.1
Cost of products sold (exclusive of items shown separately below)	1,366.0	1,486.2	4,280.4	4,462.6
Selling and administrative expenses	71.0	79.6	222.9	235.8
Depreciation	47.4	55.0	147.4	164.8
Ashland Works closure	0.0	0.0	64.1	0.0
Total operating costs	1,484.4	1,620.8	4,714.8	4,863.2
Operating profit (loss)	51.1	114.8	198.9	277.9
Interest expense	37.0	37.8	112.0	113.3
Pension and OPEB (income) expense	(6.8)	(10.0)	(7.2)	(30.0)
Other (income) expense	0.7	0.7	(16.5)	(3.4)
Income (loss) before income taxes	20.2	86.3	110.6	198.0
Income tax expense (benefit)	0.3	1.4	2.7	(4.0)
Equity in net income (loss) of subsidiaries	0.0	0.0	0.0	0.0
Net income	19.9	84.9	107.9	202.0
Less: Net income attributable to noncontrolling interests	17.1	17.7	42.8	49.5
Net income attributable to AK Steel Holding Corporation	2.8	67.2	65.1	152.5
Other comprehensive income (loss)	(5.4)	(0.9)	(37.3)	(18.3)
Comprehensive income (loss) attributable to AK Steel Holding Corporation	(2.6)	66.3	27.8	134.2
Current assets:
Cash and cash equivalents	30.8		30.8		$ 48.6
Accounts receivable, net	624.4		624.4		635.8
Inventory	1,362.7		1,362.7		1,419.9
Other current assets	48.3		48.3		97.0
Total current assets	2,066.2		2,066.2		2,201.3
Property, plant and equipment	7,094.0		7,094.0		6,969.2
Accumulated depreciation	(5,201.8)		(5,201.8)		(5,057.6)
Property, plant and equipment, net	1,892.2		1,892.2		1,911.6
Investment in subsidiaries	0.0		0.0		0.0
Inter-company accounts	0.0		0.0		0.0
Operating lease assets	253.4		253.4		0.0
Other non-current assets	392.8		392.8		402.8
TOTAL ASSETS	4,604.6		4,604.6		4,515.7
Current liabilities:
Accounts payable	676.0		676.0		801.0
Accrued liabilities	234.7		234.7		288.9
Current portion of operating lease liabilities	53.6		53.6		0.0
Current portion of pension and other postretirement benefit obligations	37.3		37.3		38.7
Total current liabilities	1,001.6		1,001.6		1,128.6
Non-current liabilities:
Long-term debt	1,969.7		1,969.7		1,993.7
Long-term operating lease liabilities	219.1		219.1		0.0
Pension and other postretirement benefit obligations	785.0		785.0		829.9
Inter-company accounts	0.0		0.0		0.0
Other non-current liabilities	170.5		170.5		134.0
TOTAL LIABILITIES	4,145.9		4,145.9		4,086.2
Equity [Abstract]
Total stockholders’ equity (deficit)	134.6		134.6		99.9
Noncontrolling interests	324.1		324.1		329.6
TOTAL EQUITY	458.7	437.8	458.7	437.8	429.5
TOTAL LIABILITIES AND EQUITY	4,604.6		4,604.6		4,515.7
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			192.5	249.9
Cash flows from investing activities:
Capital investments			(135.8)	(101.1)
Other investing items, net			10.0	0.2
Net cash flows from investing activities			(125.8)	(100.9)
Cash flows from financing activities:
Net borrowings (payments) under credit facility			(30.0)	(85.0)
Redemption of long-term debt			(3.8)	0.0
Inter-company activity			0.0	0.0
SunCoke Middletown distributions to noncontrolling interest owners			(48.3)	(54.0)
Other financing items, net			(2.4)	(0.7)
Net cash flows from financing activities			(84.5)	(139.7)
Net increase (decrease) in cash and cash equivalents			(17.8)	9.3
Cash and cash equivalents, beginning of period			48.6	38.0
Cash and cash equivalents, end of period	30.8	47.3	30.8	47.3
AK Holding [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	0.0	0.0	0.0	0.0
Cost of products sold (exclusive of items shown separately below)	0.0	0.0	0.0	0.0
Selling and administrative expenses	0.6	0.7	3.4	2.8
Depreciation	0.0	0.0	0.0	0.0
Ashland Works closure			0.0
Total operating costs	0.6	0.7	3.4	2.8
Operating profit (loss)	(0.6)	(0.7)	(3.4)	(2.8)
Interest expense	0.0	0.0	0.0	0.0
Pension and OPEB (income) expense	0.0	0.0	0.0	0.0
Other (income) expense	0.0	0.0	0.0	0.0
Income (loss) before income taxes	(0.6)	(0.7)	(3.4)	(2.8)
Income tax expense (benefit)	0.0	0.0	0.0	0.0
Equity in net income (loss) of subsidiaries	3.4	67.9	68.5	155.3
Net income	2.8	67.2	65.1	152.5
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income attributable to AK Steel Holding Corporation	2.8	67.2	65.1	152.5
Other comprehensive income (loss)	(5.4)	(0.9)	(37.3)	(18.3)
Comprehensive income (loss) attributable to AK Steel Holding Corporation	(2.6)	66.3	27.8	134.2
Current assets:
Cash and cash equivalents	0.0		0.0		0.0
Accounts receivable, net	0.0		0.0		0.0
Inventory	0.0		0.0		0.0
Other current assets	0.0		0.0		0.0
Total current assets	0.0		0.0		0.0
Property, plant and equipment	0.0		0.0		0.0
Accumulated depreciation	0.0		0.0		0.0
Property, plant and equipment, net	0.0		0.0		0.0
Investment in subsidiaries	(3,054.3)		(3,054.3)		(3,017.4)
Inter-company accounts	3,188.9		3,188.9		3,117.3
Operating lease assets	0.0		0.0
Other non-current assets	0.0		0.0		0.0
TOTAL ASSETS	134.6		134.6		99.9
Current liabilities:
Accounts payable	0.0		0.0		0.0
Accrued liabilities	0.0		0.0		0.0
Current portion of operating lease liabilities	0.0		0.0
Current portion of pension and other postretirement benefit obligations	0.0		0.0		0.0
Total current liabilities	0.0		0.0		0.0
Non-current liabilities:
Long-term debt	0.0		0.0		0.0
Long-term operating lease liabilities	0.0		0.0
Pension and other postretirement benefit obligations	0.0		0.0		0.0
Inter-company accounts	0.0		0.0		0.0
Other non-current liabilities	0.0		0.0		0.0
TOTAL LIABILITIES	0.0		0.0		0.0
Equity [Abstract]
Total stockholders’ equity (deficit)	134.6		134.6		99.9
Noncontrolling interests	0.0		0.0		0.0
TOTAL EQUITY	134.6		134.6		99.9
TOTAL LIABILITIES AND EQUITY	134.6		134.6		99.9
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			(2.5)	(2.1)
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	0.0
Redemption of long-term debt			0.0
Inter-company activity			2.5	2.4
SunCoke Middletown distributions to noncontrolling interest owners			0.0	0.0
Other financing items, net			0.0	(0.3)
Net cash flows from financing activities			2.5	2.1
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	0.0	0.0
AK Steel [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	1,396.0	1,602.0	4,491.9	4,704.3
Cost of products sold (exclusive of items shown separately below)	1,276.5	1,401.3	4,001.7	4,168.1
Selling and administrative expenses	68.5	79.6	215.2	233.2
Depreciation	36.4	43.8	108.2	130.6
Ashland Works closure			64.1
Total operating costs	1,381.4	1,524.7	4,389.2	4,531.9
Operating profit (loss)	14.6	77.3	102.7	172.4
Interest expense	35.0	36.4	106.7	109.3
Pension and OPEB (income) expense	(6.8)	(10.0)	(7.2)	(30.0)
Other (income) expense	5.8	0.6	0.9	6.4
Income (loss) before income taxes	(19.4)	50.3	2.3	86.7
Income tax expense (benefit)	(6.2)	(3.9)	(16.9)	(21.8)
Equity in net income (loss) of subsidiaries	16.6	13.7	49.3	46.8
Net income	3.4	67.9	68.5	155.3
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income attributable to AK Steel Holding Corporation	3.4	67.9	68.5	155.3
Other comprehensive income (loss)	(5.4)	(0.9)	(37.3)	(18.3)
Comprehensive income (loss) attributable to AK Steel Holding Corporation	(2.0)	67.0	31.2	137.0
Current assets:
Cash and cash equivalents	14.7		14.7		22.1
Accounts receivable, net	493.5		493.5		515.4
Inventory	1,234.7		1,234.7		1,299.6
Other current assets	38.7		38.7		85.5
Total current assets	1,781.6		1,781.6		1,922.6
Property, plant and equipment	6,193.0		6,193.0		6,111.1
Accumulated depreciation	(4,893.2)		(4,893.2)		(4,785.5)
Property, plant and equipment, net	1,299.8		1,299.8		1,325.6
Investment in subsidiaries	2,003.3		2,003.3		1,931.1
Inter-company accounts	0.0		0.0		0.0
Operating lease assets	206.0		206.0
Other non-current assets	51.1		51.1		54.3
TOTAL ASSETS	5,341.8		5,341.8		5,233.6
Current liabilities:
Accounts payable	591.9		591.9		722.5
Accrued liabilities	207.4		207.4		251.5
Current portion of operating lease liabilities	49.1		49.1
Current portion of pension and other postretirement benefit obligations	37.0		37.0		38.4
Total current liabilities	885.4		885.4		1,012.4
Non-current liabilities:
Long-term debt	1,969.7		1,969.7		1,993.7
Long-term operating lease liabilities	174.9		174.9
Pension and other postretirement benefit obligations	782.4		782.4		827.0
Inter-company accounts	4,443.5		4,443.5		4,312.3
Other non-current liabilities	140.2		140.2		105.6
TOTAL LIABILITIES	8,396.1		8,396.1		8,251.0
Equity [Abstract]
Total stockholders’ equity (deficit)	(3,054.3)		(3,054.3)		(3,017.4)
Noncontrolling interests	0.0		0.0		0.0
TOTAL EQUITY	(3,054.3)		(3,054.3)		(3,017.4)
TOTAL LIABILITIES AND EQUITY	5,341.8		5,341.8		5,233.6
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			77.6	133.1
Cash flows from investing activities:
Capital investments			(97.8)	(82.6)
Other investing items, net			9.7	0.7
Net cash flows from investing activities			(88.1)	(81.9)
Cash flows from financing activities:
Net borrowings (payments) under credit facility			(30.0)	(85.0)
Redemption of long-term debt			(3.8)
Inter-company activity			39.3	44.9
SunCoke Middletown distributions to noncontrolling interest owners			0.0	0.0
Other financing items, net			(2.4)	(0.4)
Net cash flows from financing activities			3.1	(40.5)
Net increase (decrease) in cash and cash equivalents			(7.4)	10.7
Cash and cash equivalents, beginning of period			22.1	14.5
Cash and cash equivalents, end of period	14.7	25.2	14.7	25.2
Guarantor Subsidiaries [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	76.1	81.0	239.9	242.5
Cost of products sold (exclusive of items shown separately below)	52.1	54.8	168.3	166.8
Selling and administrative expenses	3.6	3.6	10.8	11.0
Depreciation	2.1	2.0	6.4	6.0
Ashland Works closure			0.0
Total operating costs	57.8	60.4	185.5	183.8
Operating profit (loss)	18.3	20.6	54.4	58.7
Interest expense	0.0	0.0	0.0	0.0
Pension and OPEB (income) expense	0.0	0.0		0.0
Other (income) expense	(4.6)	(4.3)	(19.2)	(11.8)
Income (loss) before income taxes	22.9	24.9	73.6	70.5
Income tax expense (benefit)	5.7	6.2	18.4	17.6
Equity in net income (loss) of subsidiaries	0.0	0.0	0.0	0.0
Net income	17.2	18.7	55.2	52.9
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income attributable to AK Steel Holding Corporation	17.2	18.7	55.2	52.9
Other comprehensive income (loss)	0.0	0.0	0.0	0.0
Comprehensive income (loss) attributable to AK Steel Holding Corporation	17.2	18.7	55.2	52.9
Current assets:
Cash and cash equivalents	0.1		0.1		8.2
Accounts receivable, net	35.7		35.7		34.0
Inventory	59.4		59.4		53.9
Other current assets	0.2		0.2		0.1
Total current assets	95.4		95.4		96.2
Property, plant and equipment	196.4		196.4		189.7
Accumulated depreciation	(109.2)		(109.2)		(102.8)
Property, plant and equipment, net	87.2		87.2		86.9
Investment in subsidiaries	0.0		0.0		0.0
Inter-company accounts	1,709.9		1,709.9		1,630.7
Operating lease assets	2.8		2.8
Other non-current assets	33.3		33.3		32.9
TOTAL ASSETS	1,928.6		1,928.6		1,846.7
Current liabilities:
Accounts payable	27.4		27.4		26.2
Accrued liabilities	7.7		7.7		8.7
Current portion of operating lease liabilities	1.0		1.0
Current portion of pension and other postretirement benefit obligations	0.0		0.0		0.0
Total current liabilities	36.1		36.1		34.9
Non-current liabilities:
Long-term debt	0.0		0.0		0.0
Long-term operating lease liabilities	1.7		1.7
Pension and other postretirement benefit obligations	0.0		0.0		0.0
Inter-company accounts	0.0		0.0		0.0
Other non-current liabilities	0.6		0.6		0.2
TOTAL LIABILITIES	38.4		38.4		35.1
Equity [Abstract]
Total stockholders’ equity (deficit)	1,890.2		1,890.2		1,811.6
Noncontrolling interests	0.0		0.0		0.0
TOTAL EQUITY	1,890.2		1,890.2		1,811.6
TOTAL LIABILITIES AND EQUITY	1,928.6		1,928.6		1,846.7
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			49.2	47.6
Cash flows from investing activities:
Capital investments			(1.5)	(3.7)
Other investing items, net			0.0	0.0
Net cash flows from investing activities			(1.5)	(3.7)
Cash flows from financing activities:
Net borrowings (payments) under credit facility			0.0	0.0
Redemption of long-term debt			0.0
Inter-company activity			(55.8)	(48.9)
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			(55.8)	(48.9)
Net increase (decrease) in cash and cash equivalents			(8.1)	(5.0)
Cash and cash equivalents, beginning of period			8.2	7.2
Cash and cash equivalents, end of period	0.1	2.2	0.1	2.2
Other Non-Guarantor Subsidiaries [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	186.3	179.0	557.0	582.8
Cost of products sold (exclusive of items shown separately below)	147.1	145.7	446.4	477.1
Selling and administrative expenses	9.7	8.8	29.1	26.5
Depreciation	8.9	9.2	32.8	28.2
Ashland Works closure			0.0
Total operating costs	165.7	163.7	508.3	531.8
Operating profit (loss)	20.6	15.3	48.7	51.0
Interest expense	2.0	1.4	5.3	4.0
Pension and OPEB (income) expense	0.0	0.0		0.0
Other (income) expense	(0.4)	(0.4)	(2.2)	(2.8)
Income (loss) before income taxes	19.0	14.3	45.6	49.8
Income tax expense (benefit)	1.2	(0.3)	3.1	1.7
Equity in net income (loss) of subsidiaries	0.2	0.5	3.0	0.7
Net income	18.0	15.1	45.5	48.8
Less: Net income attributable to noncontrolling interests	17.1	17.7	42.8	49.5
Net income attributable to AK Steel Holding Corporation	0.9	(2.6)	2.7	(0.7)
Other comprehensive income (loss)	(1.4)	(0.3)	(1.7)	(1.4)
Comprehensive income (loss) attributable to AK Steel Holding Corporation	(0.5)	(2.9)	1.0	(2.1)
Current assets:
Cash and cash equivalents	16.0		16.0		18.3
Accounts receivable, net	102.4		102.4		95.3
Inventory	81.3		81.3		75.6
Other current assets	9.4		9.4		11.4
Total current assets	209.1		209.1		200.6
Property, plant and equipment	704.6		704.6		668.4
Accumulated depreciation	(199.4)		(199.4)		(169.3)
Property, plant and equipment, net	505.2		505.2		499.1
Investment in subsidiaries	71.2		71.2		68.2
Inter-company accounts	0.0		0.0		0.0
Operating lease assets	44.6		44.6
Other non-current assets	308.4		308.4		315.6
TOTAL ASSETS	1,138.5		1,138.5		1,083.5
Current liabilities:
Accounts payable	60.7		60.7		55.5
Accrued liabilities	19.6		19.6		28.7
Current portion of operating lease liabilities	3.5		3.5
Current portion of pension and other postretirement benefit obligations	0.3		0.3		0.3
Total current liabilities	84.1		84.1		84.5
Non-current liabilities:
Long-term debt	0.0		0.0		0.0
Long-term operating lease liabilities	42.5		42.5
Pension and other postretirement benefit obligations	2.6		2.6		2.9
Inter-company accounts	529.9		529.9		511.2
Other non-current liabilities	29.7		29.7		28.2
TOTAL LIABILITIES	688.8		688.8		626.8
Equity [Abstract]
Total stockholders’ equity (deficit)	125.6		125.6		127.1
Noncontrolling interests	324.1		324.1		329.6
TOTAL EQUITY	449.7		449.7		456.7
TOTAL LIABILITIES AND EQUITY	1,138.5		1,138.5		1,083.5
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			72.8	65.3
Cash flows from investing activities:
Capital investments			(36.5)	(14.8)
Other investing items, net			0.3	(0.5)
Net cash flows from investing activities			(36.2)	(15.3)
Cash flows from financing activities:
Net borrowings (payments) under credit facility			0.0	0.0
Redemption of long-term debt			0.0
Inter-company activity			9.4	7.6
SunCoke Middletown distributions to noncontrolling interest owners			(48.3)	(54.0)
Other financing items, net			0.0	0.0
Net cash flows from financing activities			(38.9)	(46.4)
Net increase (decrease) in cash and cash equivalents			(2.3)	3.6
Cash and cash equivalents, beginning of period			18.3	16.3
Cash and cash equivalents, end of period	16.0	19.9	16.0	19.9
Consolidation, Eliminations [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	(122.9)	(126.4)	(375.1)	(388.5)
Cost of products sold (exclusive of items shown separately below)	(109.7)	(115.6)	(336.0)	(349.4)
Selling and administrative expenses	(11.4)	(13.1)	(35.6)	(37.7)
Depreciation	0.0	0.0	0.0	0.0
Ashland Works closure			0.0
Total operating costs	(121.1)	(128.7)	(371.6)	(387.1)
Operating profit (loss)	(1.8)	2.3	(3.5)	(1.4)
Interest expense	0.0	0.0	0.0	0.0
Pension and OPEB (income) expense	0.0	0.0		0.0
Other (income) expense	(0.1)	4.8	4.0	4.8
Income (loss) before income taxes	(1.7)	(2.5)	(7.5)	(6.2)
Income tax expense (benefit)	(0.4)	(0.6)	(1.9)	(1.5)
Equity in net income (loss) of subsidiaries	(20.2)	(82.1)	(120.8)	(202.8)
Net income	(21.5)	(84.0)	(126.4)	(207.5)
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income attributable to AK Steel Holding Corporation	(21.5)	(84.0)	(126.4)	(207.5)
Other comprehensive income (loss)	6.8	1.2	39.0	19.7
Comprehensive income (loss) attributable to AK Steel Holding Corporation	(14.7)	(82.8)	(87.4)	(187.8)
Current assets:
Cash and cash equivalents	0.0		0.0		0.0
Accounts receivable, net	(7.2)		(7.2)		(8.9)
Inventory	(12.7)		(12.7)		(9.2)
Other current assets	0.0		0.0		0.0
Total current assets	(19.9)		(19.9)		(18.1)
Property, plant and equipment	0.0		0.0		0.0
Accumulated depreciation	0.0		0.0		0.0
Property, plant and equipment, net	0.0		0.0		0.0
Investment in subsidiaries	979.8		979.8		1,018.1
Inter-company accounts	(4,898.8)		(4,898.8)		(4,748.0)
Operating lease assets	0.0		0.0
Other non-current assets	0.0		0.0		0.0
TOTAL ASSETS	(3,938.9)		(3,938.9)		(3,748.0)
Current liabilities:
Accounts payable	(4.0)		(4.0)		(3.2)
Accrued liabilities	0.0		0.0		0.0
Current portion of operating lease liabilities	0.0		0.0
Current portion of pension and other postretirement benefit obligations	0.0		0.0		0.0
Total current liabilities	(4.0)		(4.0)		(3.2)
Non-current liabilities:
Long-term debt	0.0		0.0		0.0
Long-term operating lease liabilities	0.0		0.0
Pension and other postretirement benefit obligations	0.0		0.0		0.0
Inter-company accounts	(4,973.4)		(4,973.4)		(4,823.5)
Other non-current liabilities	0.0		0.0		0.0
TOTAL LIABILITIES	(4,977.4)		(4,977.4)		(4,826.7)
Equity [Abstract]
Total stockholders’ equity (deficit)	1,038.5		1,038.5		1,078.7
Noncontrolling interests	0.0		0.0		0.0
TOTAL EQUITY	1,038.5		1,038.5		1,078.7
TOTAL LIABILITIES AND EQUITY	(3,938.9)		(3,938.9)		$ (3,748.0)
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			(4.6)	6.0
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	0.0
Redemption of long-term debt			0.0
Inter-company activity			4.6	(6.0)
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			4.6	(6.0)
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	$ 0.0	$ 0.0