Supplementary Guarantor Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Statements of Operations [Abstract]
Net sales	$ 1,445.7	$ 1,535.5	$ 1,680.5	$ 1,697.7	$ 1,677.1	$ 1,735.6	$ 1,746.6	$ 1,658.9	$ 6,359.4	$ 6,818.2	$ 6,080.5
Cost of products sold (exclusive of items shown separately below)									5,606.3	5,911.0	5,253.1
Selling and administrative expenses									295.2	322.6	284.9
Depreciation									192.6	220.2	226.0
Ashland Works closure									56.0	0.0	0.0
Asset impairment charge									0.0	0.0	75.6
Credit for adjustment of liability for transportation costs									0.0	0.0	(19.3)
Total operating costs									6,150.1	6,453.8	5,820.3
Operating profit (loss)	10.4	51.1	106.6	41.2	86.5	114.8	99.5	63.6	209.3	364.4	260.2
Interest expense									146.6	151.6	152.3
Pension and OPEB (income) expense									12.0	(19.2)	(71.9)
Other (income) expense									(18.5)	(5.9)	17.1
Income (loss) before income taxes									69.2	237.9	162.7
Income tax expense (benefit)									6.2	(6.2)	(2.2)
Equity in net income (loss) of subsidiaries									0.0	0.0	0.0
Net income									63.0	244.1	164.9
Less: Net income attributable to noncontrolling interests									51.8	58.1	61.4
Net income attributable to AK Steel Holding Corporation	(53.9)	$ 2.8	$ 66.8	(4.5)	33.5	$ 67.2	$ 56.6	28.7	11.2	186.0	103.5
Other comprehensive income (loss)									32.0	(50.6)	38.5
Comprehensive income attributable to AK Steel Holding Corporation									43.2	135.4	142.0
Current assets:
Cash and cash equivalents	31.0			48.6	48.6			38.0	31.0	48.6	173.2	$ 31.0	$ 48.6	$ 38.0
Accounts receivable, net												577.9	635.8
Inventory												1,346.2	1,419.9
Other current assets												65.2	97.0
Total current assets												2,020.3	2,201.3
Property, plant and equipment												7,168.9	6,969.2
Accumulated depreciation												(5,237.0)	(5,057.6)
Property, plant and equipment, net												1,931.9	1,911.6
Other non-current assets:
Investment in subsidiaries												0.0	0.0
Inter-company accounts												0.0	0.0
Goodwill and intangible assets												293.4	298.9
Other non-current assets												345.0	103.9
TOTAL ASSETS												4,590.6	4,515.7
Current liabilities:
Accounts payable												705.1	801.0
Accrued liabilities												314.4	288.9
Current portion of pension and other postretirement benefit obligations												41.0	38.7
Total current liabilities												1,060.5	1,128.6
Non-current liabilities:
Long-term debt												1,968.8	1,993.7
Pension and other postretirement benefit obligations												717.8	829.9
Inter-company accounts												0.0	0.0
Other non-current liabilities												366.2	134.0
TOTAL LIABILITIES												4,113.3	4,086.2
Equity (deficit):
Total stockholders’ equity (deficit)												151.5	99.9
Noncontrolling interests												325.8	329.6
TOTAL EQUITY (DEFICIT)												477.3	429.5	300.6
TOTAL LIABILITIES AND EQUITY (DEFICIT)												4,590.6	4,515.7
Condensed Cash Flows [Abstract]
Net cash flows from operating activities									265.9	364.7	198.8
Cash flows from investing activities:
Capital investments									(194.8)	(152.0)	(152.5)
Investment in acquired business, net of cash acquired									0.0	0.0	(360.4)
Other investing items, net									6.6	0.1	4.2
Net cash flows from investing activities									(188.2)	(151.9)	(508.7)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Net borrowings (repayments) under credit facility									115.0	(115.0)	450.0
Proceeds from issuance of long-term debt									0.0	0.0	680.0
Redemption of long-term debt									(152.3)	(12.6)	(848.4)
Debt issuance costs									0.0	0.0	(25.3)
Inter-company activity									0.0	0.0	0.0
SunCoke Middletown distributions to noncontrolling interest owners									(55.6)	(73.7)	(79.1)
Other financing items, net									(2.4)	(0.9)	(2.5)
Net cash flows from financing activities									(95.3)	(202.2)	174.7
Net increase (decrease) in cash and cash equivalents									(17.6)	10.6	(135.2)
Cash and cash equivalents, beginning of year				48.6				38.0	48.6	38.0	173.2
Cash and cash equivalents, end of year	31.0				48.6				31.0	48.6	38.0
Parent Company [Member]
Condensed Statements of Operations [Abstract]
Net sales									0.0	0.0	0.0
Cost of products sold (exclusive of items shown separately below)									0.0	0.0	0.0
Selling and administrative expenses									4.3	3.5	3.7
Depreciation									0.0	0.0	0.0
Ashland Works closure									0.0
Asset impairment charge											0.0
Credit for adjustment of liability for transportation costs											0.0
Total operating costs									4.3	3.5	3.7
Operating profit (loss)									(4.3)	(3.5)	(3.7)
Interest expense									0.0	0.0	0.0
Pension and OPEB (income) expense									0.0	0.0	0.0
Other (income) expense									0.0	0.0	0.0
Income (loss) before income taxes									(4.3)	(3.5)	(3.7)
Income tax expense (benefit)									0.0	0.0	0.0
Equity in net income (loss) of subsidiaries									15.5	189.5	107.2
Net income									11.2	186.0	103.5
Less: Net income attributable to noncontrolling interests									0.0	0.0	0.0
Net income attributable to AK Steel Holding Corporation									11.2	186.0	103.5
Other comprehensive income (loss)									32.0	(50.6)	38.5
Comprehensive income attributable to AK Steel Holding Corporation									43.2	135.4	142.0
Current assets:
Cash and cash equivalents	0.0			0.0	0.0			0.0	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
Other non-current assets:
Investment in subsidiaries												(3,108.8)	(3,017.4)
Inter-company accounts												3,260.3	3,117.3
Goodwill and intangible assets												0.0	0.0
Other non-current assets												0.0	0.0
TOTAL ASSETS												151.5	99.9
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 (deficit):
Total stockholders’ equity (deficit)												151.5	99.9
Noncontrolling interests												0.0	0.0
TOTAL EQUITY (DEFICIT)												151.5	99.9
TOTAL LIABILITIES AND EQUITY (DEFICIT)												151.5	99.9
Condensed Cash Flows [Abstract]
Net cash flows from operating activities									(3.2)	(2.3)	(2.5)
Cash flows from investing activities:
Capital investments									0.0	0.0	0.0
Investment in acquired business, net of cash acquired											0.0
Other investing items, net									0.0	0.0	0.0
Net cash flows from investing activities									0.0	0.0	0.0
Net Cash Provided by (Used in) Financing Activities [Abstract]
Net borrowings (repayments) under credit facility									0.0	0.0	0.0
Proceeds from issuance of long-term debt											0.0
Redemption of long-term debt									0.0	0.0	0.0
Debt issuance costs											0.0
Inter-company activity									3.9	2.5	5.0
SunCoke Middletown distributions to noncontrolling interest owners									0.0	0.0	0.0
Other financing items, net									(0.7)	(0.2)	(2.5)
Net cash flows from financing activities									3.2	2.3	2.5
Net increase (decrease) in cash and cash equivalents									0.0	0.0	0.0
Cash and cash equivalents, beginning of year				0.0				0.0	0.0	0.0	0.0
Cash and cash equivalents, end of year	0.0				0.0				0.0	0.0	0.0
AK Steel [Member]
Condensed Statements of Operations [Abstract]
Net sales									5,784.3	6,244.7	5,755.1
Cost of products sold (exclusive of items shown separately below)									5,228.5	5,519.8	5,082.0
Selling and administrative expenses									283.0	320.0	285.5
Depreciation									142.1	171.5	189.3
Ashland Works closure									56.0
Asset impairment charge											75.6
Credit for adjustment of liability for transportation costs											(19.3)
Total operating costs									5,709.6	6,011.3	5,613.1
Operating profit (loss)									74.7	233.4	142.0
Interest expense									139.3	145.9	150.3
Pension and OPEB (income) expense									12.0	(19.2)	(71.9)
Other (income) expense									2.6	8.7	30.1
Income (loss) before income taxes									(79.2)	98.0	33.5
Income tax expense (benefit)									(25.4)	(26.5)	(29.4)
Equity in net income (loss) of subsidiaries									69.3	65.0	44.3
Net income									15.5	189.5	107.2
Less: Net income attributable to noncontrolling interests									0.0	0.0	0.0
Net income attributable to AK Steel Holding Corporation									15.5	189.5	107.2
Other comprehensive income (loss)									32.0	(50.6)	38.5
Comprehensive income attributable to AK Steel Holding Corporation									47.5	138.9	145.7
Current assets:
Cash and cash equivalents	11.8			22.1	22.1			14.5	22.1	14.5	14.5	11.8	22.1	14.5
Accounts receivable, net												437.4	515.4
Inventory												1,214.9	1,299.6
Other current assets												59.1	85.5
Total current assets												1,723.2	1,922.6
Property, plant and equipment												6,234.4	6,111.1
Accumulated depreciation												(4,917.2)	(4,785.5)
Property, plant and equipment, net												1,317.2	1,325.6
Other non-current assets:
Investment in subsidiaries												2,023.7	1,931.1
Inter-company accounts												0.0	0.0
Goodwill and intangible assets												0.0	0.0
Other non-current assets												248.6	54.3
TOTAL ASSETS												5,312.7	5,233.6
Current liabilities:
Accounts payable												614.2	722.5
Accrued liabilities												276.0	251.5
Current portion of pension and other postretirement benefit obligations												40.7	38.4
Total current liabilities												930.9	1,012.4
Non-current liabilities:
Long-term debt												1,968.8	1,993.7
Pension and other postretirement benefit obligations												714.8	827.0
Inter-company accounts												4,513.6	4,312.3
Other non-current liabilities												293.4	105.6
TOTAL LIABILITIES												8,421.5	8,251.0
Equity (deficit):
Total stockholders’ equity (deficit)												(3,108.8)	(3,017.4)
Noncontrolling interests												0.0	0.0
TOTAL EQUITY (DEFICIT)												(3,108.8)	(3,017.4)
TOTAL LIABILITIES AND EQUITY (DEFICIT)												5,312.7	5,233.6
Condensed Cash Flows [Abstract]
Net cash flows from operating activities									114.6	209.7	67.3
Cash flows from investing activities:
Capital investments									(133.5)	(122.9)	(131.8)
Investment in acquired business, net of cash acquired											(360.4)
Other investing items, net									6.6	0.8	4.0
Net cash flows from investing activities									(126.9)	(122.1)	(488.2)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Net borrowings (repayments) under credit facility									115.0	(115.0)	450.0
Proceeds from issuance of long-term debt											680.0
Redemption of long-term debt									(152.3)	(12.6)	(848.4)
Debt issuance costs											(25.3)
Inter-company activity									41.0	48.3	31.2
SunCoke Middletown distributions to noncontrolling interest owners									0.0	0.0	0.0
Other financing items, net									(1.7)	(0.7)	0.0
Net cash flows from financing activities									2.0	(80.0)	287.5
Net increase (decrease) in cash and cash equivalents									(10.3)	7.6	(133.4)
Cash and cash equivalents, beginning of year				22.1				14.5	22.1	14.5	147.9
Cash and cash equivalents, end of year	11.8				22.1				11.8	22.1	14.5
Guarantor Subsidiaries of the Senior Notes [Member]
Condensed Statements of Operations [Abstract]
Net sales									304.1	320.8	285.9
Cost of products sold (exclusive of items shown separately below)									212.9	221.4	197.7
Selling and administrative expenses									13.8	14.5	13.5
Depreciation									8.5	8.0	7.5
Ashland Works closure									0.0
Asset impairment charge											0.0
Credit for adjustment of liability for transportation costs											0.0
Total operating costs									235.2	243.9	218.7
Operating profit (loss)									68.9	76.9	67.2
Interest expense									0.0	0.0	0.0
Pension and OPEB (income) expense									0.0	0.0	0.0
Other (income) expense									(23.2)	(16.4)	(11.4)
Income (loss) before income taxes									92.1	93.3	78.6
Income tax expense (benefit)									23.0	23.3	29.9
Equity in net income (loss) of subsidiaries									0.0	0.0	0.0
Net income									69.1	70.0	48.7
Less: Net income attributable to noncontrolling interests									0.0	0.0	0.0
Net income attributable to AK Steel Holding Corporation									69.1	70.0	48.7
Other comprehensive income (loss)									0.0	0.0	0.0
Comprehensive income attributable to AK Steel Holding Corporation									69.1	70.0	48.7
Current assets:
Cash and cash equivalents	0.1			8.2	8.2			7.2	8.2	7.2	7.2	0.1	8.2	7.2
Accounts receivable, net												25.7	34.0
Inventory												56.0	53.9
Other current assets												0.2	0.1
Total current assets												82.0	96.2
Property, plant and equipment												199.5	189.7
Accumulated depreciation												(111.3)	(102.8)
Property, plant and equipment, net												88.2	86.9
Other non-current assets:
Investment in subsidiaries												0.0	0.0
Inter-company accounts												1,734.6	1,630.7
Goodwill and intangible assets												32.8	32.9
Other non-current assets												2.8	0.0
TOTAL ASSETS												1,940.4	1,846.7
Current liabilities:
Accounts payable												25.4	26.2
Accrued liabilities												8.8	8.7
Current portion of pension and other postretirement benefit obligations												0.0	0.0
Total current liabilities												34.2	34.9
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												2.1	0.2
TOTAL LIABILITIES												36.3	35.1
Equity (deficit):
Total stockholders’ equity (deficit)												1,904.1	1,811.6
Noncontrolling interests												0.0	0.0
TOTAL EQUITY (DEFICIT)												1,904.1	1,811.6
TOTAL LIABILITIES AND EQUITY (DEFICIT)												1,940.4	1,846.7
Condensed Cash Flows [Abstract]
Net cash flows from operating activities									76.2	83.1	73.3
Cash flows from investing activities:
Capital investments									(3.8)	(8.1)	(5.6)
Investment in acquired business, net of cash acquired											0.0
Other investing items, net									0.0	0.0	0.0
Net cash flows from investing activities									(3.8)	(8.1)	(5.6)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Net borrowings (repayments) under credit facility									0.0	0.0	0.0
Proceeds from issuance of long-term debt											0.0
Redemption of long-term debt									0.0	0.0	0.0
Debt issuance costs											0.0
Inter-company activity									(80.5)	(74.0)	(64.9)
SunCoke Middletown distributions to noncontrolling interest owners									0.0	0.0	0.0
Other financing items, net									0.0	0.0	0.0
Net cash flows from financing activities									(80.5)	(74.0)	(64.9)
Net increase (decrease) in cash and cash equivalents									(8.1)	1.0	2.8
Cash and cash equivalents, beginning of year				8.2				7.2	8.2	7.2	4.4
Cash and cash equivalents, end of year	0.1				8.2				0.1	8.2	7.2
Other Non-Guarantor Subsidiaries [Member]
Condensed Statements of Operations [Abstract]
Net sales									751.5	752.8	496.3
Cost of products sold (exclusive of items shown separately below)									598.5	620.9	387.2
Selling and administrative expenses									40.0	34.2	26.9
Depreciation									42.0	40.7	29.2
Ashland Works closure									0.0
Asset impairment charge											0.0
Credit for adjustment of liability for transportation costs											0.0
Total operating costs									680.5	695.8	443.3
Operating profit (loss)									71.0	57.0	53.0
Interest expense									7.3	5.7	2.0
Pension and OPEB (income) expense									0.0	0.0	0.0
Other (income) expense									(4.1)	(2.9)	(5.4)
Income (loss) before income taxes									67.8	54.2	56.4
Income tax expense (benefit)									10.5	(2.0)	(1.8)
Equity in net income (loss) of subsidiaries									1.9	0.7	0.0
Net income									59.2	56.9	58.2
Less: Net income attributable to noncontrolling interests									51.8	58.1	61.4
Net income attributable to AK Steel Holding Corporation									7.4	(1.2)	(3.2)
Other comprehensive income (loss)									(0.6)	(1.7)	4.7
Comprehensive income attributable to AK Steel Holding Corporation									6.8	(2.9)	1.5
Current assets:
Cash and cash equivalents	19.1			18.3	18.3			16.3	19.1	16.3	20.9	19.1	18.3	16.3
Accounts receivable, net												124.1	95.3
Inventory												85.5	75.6
Other current assets												5.9	11.4
Total current assets												234.6	200.6
Property, plant and equipment												735.0	668.4
Accumulated depreciation												(208.5)	(169.3)
Property, plant and equipment, net												526.5	499.1
Other non-current assets:
Investment in subsidiaries												72.0	68.2
Inter-company accounts												0.0	0.0
Goodwill and intangible assets												260.6	266.0
Other non-current assets												93.6	49.6
TOTAL ASSETS												1,187.3	1,083.5
Current liabilities:
Accounts payable												70.1	55.5
Accrued liabilities												29.6	28.7
Current portion of pension and other postretirement benefit obligations												0.3	0.3
Total current liabilities												100.0	84.5
Non-current liabilities:
Long-term debt												0.0	0.0
Pension and other postretirement benefit obligations												3.0	2.9
Inter-company accounts												558.6	511.2
Other non-current liabilities												70.7	28.2
TOTAL LIABILITIES												732.3	626.8
Equity (deficit):
Total stockholders’ equity (deficit)												129.2	127.1
Noncontrolling interests												325.8	329.6
TOTAL EQUITY (DEFICIT)												455.0	456.7
TOTAL LIABILITIES AND EQUITY (DEFICIT)												1,187.3	1,083.5
Condensed Cash Flows [Abstract]
Net cash flows from operating activities									83.6	83.8	61.7
Cash flows from investing activities:
Capital investments									(57.5)	(21.0)	(15.1)
Investment in acquired business, net of cash acquired											0.0
Other investing items, net									0.0	(0.7)	0.2
Net cash flows from investing activities									(57.5)	(21.7)	(14.9)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Net borrowings (repayments) under credit facility									0.0	0.0	0.0
Proceeds from issuance of long-term debt											0.0
Redemption of long-term debt									0.0	0.0	0.0
Debt issuance costs											0.0
Inter-company activity									30.3	13.6	27.7
SunCoke Middletown distributions to noncontrolling interest owners									(55.6)	(73.7)	(79.1)
Other financing items, net									0.0	0.0	0.0
Net cash flows from financing activities									(25.3)	(60.1)	(51.4)
Net increase (decrease) in cash and cash equivalents									0.8	2.0	(4.6)
Cash and cash equivalents, beginning of year				18.3				16.3	18.3	16.3	20.9
Cash and cash equivalents, end of year	19.1				18.3				19.1	18.3	16.3
Eliminations [Member]
Condensed Statements of Operations [Abstract]
Net sales									(480.5)	(500.1)	(456.8)
Cost of products sold (exclusive of items shown separately below)									(433.6)	(451.1)	(413.8)
Selling and administrative expenses									(45.9)	(49.6)	(44.7)
Depreciation									0.0	0.0	0.0
Ashland Works closure									0.0
Asset impairment charge											0.0
Credit for adjustment of liability for transportation costs											0.0
Total operating costs									(479.5)	(500.7)	(458.5)
Operating profit (loss)									(1.0)	0.6	1.7
Interest expense									0.0	0.0	0.0
Pension and OPEB (income) expense									0.0	0.0	0.0
Other (income) expense									6.2	4.7	3.8
Income (loss) before income taxes									(7.2)	(4.1)	(2.1)
Income tax expense (benefit)									(1.9)	(1.0)	(0.9)
Equity in net income (loss) of subsidiaries									(86.7)	(255.2)	(151.5)
Net income									(92.0)	(258.3)	(152.7)
Less: Net income attributable to noncontrolling interests									0.0	0.0	0.0
Net income attributable to AK Steel Holding Corporation									(92.0)	(258.3)	(152.7)
Other comprehensive income (loss)									(31.4)	52.3	(43.2)
Comprehensive income attributable to AK Steel Holding Corporation									(123.4)	(206.0)	(195.9)
Current assets:
Cash and cash equivalents	0.0			0.0	0.0			0.0	0.0	0.0	0.0	0.0	0.0	$ 0.0
Accounts receivable, net												(9.3)	(8.9)
Inventory												(10.2)	(9.2)
Other current assets												0.0	0.0
Total current assets												(19.5)	(18.1)
Property, plant and equipment												0.0	0.0
Accumulated depreciation												0.0	0.0
Property, plant and equipment, net												0.0	0.0
Other non-current assets:
Investment in subsidiaries												1,013.1	1,018.1
Inter-company accounts												(4,994.9)	(4,748.0)
Goodwill and intangible assets												0.0	0.0
Other non-current assets												0.0	0.0
TOTAL ASSETS												(4,001.3)	(3,748.0)
Current liabilities:
Accounts payable												(4.6)	(3.2)
Accrued liabilities												0.0	0.0
Current portion of pension and other postretirement benefit obligations												0.0	0.0
Total current liabilities												(4.6)	(3.2)
Non-current liabilities:
Long-term debt												0.0	0.0
Pension and other postretirement benefit obligations												0.0	0.0
Inter-company accounts												(5,072.2)	(4,823.5)
Other non-current liabilities												0.0	0.0
TOTAL LIABILITIES												(5,076.8)	(4,826.7)
Equity (deficit):
Total stockholders’ equity (deficit)												1,075.5	1,078.7
Noncontrolling interests												0.0	0.0
TOTAL EQUITY (DEFICIT)												1,075.5	1,078.7
TOTAL LIABILITIES AND EQUITY (DEFICIT)												$ (4,001.3)	$ (3,748.0)
Condensed Cash Flows [Abstract]
Net cash flows from operating activities									(5.3)	(9.6)	(1.0)
Cash flows from investing activities:
Capital investments									0.0	0.0	0.0
Investment in acquired business, net of cash acquired											0.0
Other investing items, net									0.0	0.0	0.0
Net cash flows from investing activities									0.0	0.0	0.0
Net Cash Provided by (Used in) Financing Activities [Abstract]
Net borrowings (repayments) under credit facility									0.0	0.0	0.0
Proceeds from issuance of long-term debt											0.0
Redemption of long-term debt									0.0	0.0	0.0
Debt issuance costs											0.0
Inter-company activity									5.3	9.6	1.0
SunCoke Middletown distributions to noncontrolling interest owners									0.0	0.0	0.0
Other financing items, net									0.0	0.0	0.0
Net cash flows from financing activities									5.3	9.6	1.0
Net increase (decrease) in cash and cash equivalents									0.0	0.0	0.0
Cash and cash equivalents, beginning of year				$ 0.0				$ 0.0	0.0	0.0	0.0
Cash and cash equivalents, end of year	$ 0.0				$ 0.0				$ 0.0	$ 0.0	$ 0.0