Supplementary Guarantor Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	$ 1,557.2	$ 1,492.2	$ 3,090.6	$ 3,011.0
Cost of products sold (exclusive of items shown separately below)	1,343.7	1,325.0	2,654.5	2,690.5
Selling and administrative expenses (exclusive of items shown separately below)	65.1	62.2	136.3	125.7
Depreciation	55.1	54.3	110.2	108.0
Pension and OPEB expense (income)	(16.2)	(11.9)	(32.4)	(23.8)
Total operating costs	1,447.7	1,429.6	2,868.6	2,900.4
Operating profit (loss)	109.5	62.6	222.0	110.6
Interest expense	38.2	41.4	77.6	84.2
Other income (expense)	(3.6)	2.1	(11.4)	1.4
Income (loss) before income taxes	67.7	23.3	133.0	27.8
Income tax expense (benefit)	(8.7)	(10.6)	(22.1)	(10.5)
Equity in net income (loss) of subsidiaries	0.0	0.0	0.0	0.0
Net income (loss)	76.4	33.9	155.1	38.3
Less: Net income attributable to noncontrolling interests	15.2	16.6	31.4	34.6
Net income (loss) attributable to AK Steel Holding Corporation	61.2	17.3	123.7	3.7
Other comprehensive income (loss)	(19.9)	42.4	(42.6)	41.4
Comprehensive income (loss) attributable to AK Steel Holding Corporation	41.3	59.7	81.1	45.1
Current assets:
Cash and cash equivalents	136.3	56.2	173.2	56.6	$ 136.3		$ 173.2	$ 56.2	$ 56.6
Accounts receivable, net					530.9		442.0
Inventory, net					1,103.5		1,113.9
Other current assets					83.7		94.6
Total current assets					1,854.4		1,823.7
Property, plant and equipment					6,609.7		6,569.0
Accumulated depreciation					(4,663.5)		(4,554.6)
Property, plant and equipment, net					1,946.2		2,014.4
Investment in subsidiaries					0.0		0.0
Inter-company accounts					0.0		0.0
Other non-current assets					205.0		197.9
TOTAL ASSETS					4,005.6		4,036.0
Current liabilities:
Accounts payable					672.8		589.9
Accrued liabilities					212.9		234.1
Current portion of pension and other postretirement benefit obligations					41.3		41.3
Total current liabilities					927.0		865.3
Non-current liabilities:
Long-term debt					1,689.7		1,816.6
Pension and other postretirement benefit obligations					1,057.1		1,093.7
Other non-current liabilities					146.2		148.4
TOTAL LIABILITIES					3,820.0		3,924.0
EXCHANGEABLE NOTES EXCHANGE FEATURE					0.0		21.3
Equity [Abstract]
Total stockholders' equity (deficit)					(166.8)		(272.2)
Noncontrolling interests					352.4		362.9
TOTAL EQUITY					185.6		90.7	(300.6)	(595.6)
TOTAL LIABILITIES AND EQUITY					4,005.6		4,036.0
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			203.2	273.0
Cash flows from investing activities:
Capital investments			(52.1)	(53.9)
Cash held for retirement of debt			0.0	(135.4)
Other investing items, net			2.9	2.1
Net cash flows from investing activities			(49.2)	(187.2)
Cash flows from financing activities:
Net borrowings (payments) under credit facility			0.0	(400.0)
Proceeds from issuance of long-term debt			400.0	380.0
Redemption of long-term debt			(538.2)	(259.0)
Proceeds from issuance of common stock			0.0	249.4
Debt issuance costs			(8.3)	(19.3)
Inter-company activity			0.0	0.0
SunCoke Middletown distributions to noncontrolling interest owners			(41.9)	(37.0)
Other financing items, net			(2.5)	(0.3)
Net cash flows from financing activities			(190.9)	(86.2)
Net increase (decrease) in cash and cash equivalents			(36.9)	(0.4)
Cash and cash equivalents, beginning of period			173.2	56.6
Cash and cash equivalents, end of period	136.3	56.2	136.3	56.2
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 (exclusive of items shown separately below)	1.0	1.4	2.4	2.7
Depreciation	0.0	0.0	0.0	0.0
Pension and OPEB expense (income)	0.0	0.0	0.0	0.0
Total operating costs	1.0	1.4	2.4	2.7
Operating profit (loss)	(1.0)	(1.4)	(2.4)	(2.7)
Interest 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	(1.0)	(1.4)	(2.4)	(2.7)
Income tax expense (benefit)	0.0	0.0	0.0	0.0
Equity in net income (loss) of subsidiaries	62.2	18.7	126.1	6.4
Net income (loss)	61.2	17.3	123.7	3.7
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income (loss) attributable to AK Steel Holding Corporation	61.2	17.3	123.7	3.7
Other comprehensive income (loss)	(19.9)	42.4	(42.6)	41.4
Comprehensive income (loss) attributable to AK Steel Holding Corporation	41.3	59.7	81.1	45.1
Current assets:
Cash and cash equivalents	0.0	0.0	0.0	0.0	0.0		0.0	0.0	0.0
Accounts receivable, net					0.0		0.0
Inventory, net					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					(3,186.2)		(3,227.1)
Inter-company accounts					3,019.4		2,954.9
Other non-current assets					0.0		0.0
TOTAL ASSETS					(166.8)		(272.2)
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
Other non-current liabilities					0.0		0.0
TOTAL LIABILITIES					0.0		0.0
EXCHANGEABLE NOTES EXCHANGE FEATURE							0.0
Equity [Abstract]
Total stockholders' equity (deficit)					(166.8)		(272.2)
Noncontrolling interests					0.0		0.0
TOTAL EQUITY					(166.8)		(272.2)
TOTAL LIABILITIES AND EQUITY					(166.8)		(272.2)
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			(1.8)	(2.0)
Cash flows from investing activities:
Capital investments			0.0	0.0
Cash held for retirement of debt				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	0.0
Redemption of long-term debt			0.0	0.0
Proceeds from issuance of common stock				249.4
Debt issuance costs			0.0	0.0
Inter-company activity			4.2	(247.1)
SunCoke Middletown distributions to noncontrolling interest owners			0.0	0.0
Other financing items, net			(2.4)	(0.3)
Net cash flows from financing activities			1.8	2.0
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
AK Steel [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	1,498.4	1,436.8	2,972.0	2,905.0
Cost of products sold (exclusive of items shown separately below)	1,321.7	1,305.5	2,608.2	2,656.8
Selling and administrative expenses (exclusive of items shown separately below)	67.8	62.7	140.1	127.2
Depreciation	46.9	46.9	94.1	93.3
Pension and OPEB expense (income)	(16.2)	(11.9)	(32.4)	(23.8)
Total operating costs	1,420.2	1,403.2	2,810.0	2,853.5
Operating profit (loss)	78.2	33.6	162.0	51.5
Interest expense	37.9	41.1	76.9	83.4
Other income (expense)	(7.8)	(5.1)	(19.2)	(9.2)
Income (loss) before income taxes	32.5	(12.6)	65.9	(41.1)
Income tax expense (benefit)	(16.6)	(18.6)	(36.5)	(24.7)
Equity in net income (loss) of subsidiaries	13.1	12.7	23.7	22.8
Net income (loss)	62.2	18.7	126.1	6.4
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income (loss) attributable to AK Steel Holding Corporation	62.2	18.7	126.1	6.4
Other comprehensive income (loss)	(19.9)	42.4	(42.6)	41.4
Comprehensive income (loss) attributable to AK Steel Holding Corporation	42.3	61.1	83.5	47.8
Current assets:
Cash and cash equivalents	111.6	20.3	111.6	27.0	111.6		147.9	20.3	27.0
Accounts receivable, net					485.2		400.4
Inventory, net					1,024.9		1,038.1
Other current assets					79.9		91.5
Total current assets					1,701.6		1,677.9
Property, plant and equipment					5,894.0		5,857.9
Accumulated depreciation					(4,459.0)		(4,365.8)
Property, plant and equipment, net					1,435.0		1,492.1
Investment in subsidiaries					1,458.0		1,406.2
Inter-company accounts					(4,149.8)		(4,041.9)
Other non-current assets					130.1		126.0
TOTAL ASSETS					574.9		660.3
Current liabilities:
Accounts payable					638.6		557.7
Accrued liabilities					193.7		213.7
Current portion of pension and other postretirement benefit obligations					41.0		41.0
Total current liabilities					873.3		812.4
Non-current liabilities:
Long-term debt					1,689.7		1,816.6
Pension and other postretirement benefit obligations					1,053.6		1,090.4
Other non-current liabilities					144.5		146.7
TOTAL LIABILITIES					3,761.1		3,866.1
EXCHANGEABLE NOTES EXCHANGE FEATURE							21.3
Equity [Abstract]
Total stockholders' equity (deficit)					(3,186.2)		(3,227.1)
Noncontrolling interests					0.0		0.0
TOTAL EQUITY					(3,186.2)		(3,227.1)
TOTAL LIABILITIES AND EQUITY					574.9		660.3
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			141.1	209.8
Cash flows from investing activities:
Capital investments			(47.9)	(49.0)
Cash held for retirement of debt				(135.4)
Other investing items, net			3.0	0.1
Net cash flows from investing activities			(44.9)	(184.3)
Cash flows from financing activities:
Net borrowings (payments) under credit facility				(400.0)
Proceeds from issuance of long-term debt			400.0	380.0
Redemption of long-term debt			(538.2)	(259.0)
Proceeds from issuance of common stock				0.0
Debt issuance costs			(8.3)	(19.3)
Inter-company activity			14.1	266.1
SunCoke Middletown distributions to noncontrolling interest owners			0.0	0.0
Other financing items, net			(0.1)	0.0
Net cash flows from financing activities			(132.5)	(32.2)
Net increase (decrease) in cash and cash equivalents			(36.3)	(6.7)
Cash and cash equivalents, beginning of period			147.9	27.0
Cash and cash equivalents, end of period	111.6	20.3	111.6	20.3
Guarantor Subsidiaries [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	74.1	64.4	146.4	125.9
Cost of products sold (exclusive of items shown separately below)	53.2	45.6	105.1	86.1
Selling and administrative expenses (exclusive of items shown separately below)	3.2	3.2	6.4	6.6
Depreciation	2.1	1.9	4.1	3.9
Pension and OPEB expense (income)	0.0	0.0	0.0	0.0
Total operating costs	58.5	50.7	115.6	96.6
Operating profit (loss)	15.6	13.7	30.8	29.3
Interest expense	0.0	0.0	0.0	0.0
Other income (expense)	2.8	2.0	5.2	4.0
Income (loss) before income taxes	18.4	15.7	36.0	33.3
Income tax expense (benefit)	7.0	6.0	13.7	12.7
Equity in net income (loss) of subsidiaries	0.0	0.0	0.0	0.0
Net income (loss)	11.4	9.7	22.3	20.6
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income (loss) attributable to AK Steel Holding Corporation	11.4	9.7	22.3	20.6
Other comprehensive income (loss)	0.0	0.0	0.0	0.0
Comprehensive income (loss) attributable to AK Steel Holding Corporation	11.4	9.7	22.3	20.6
Current assets:
Cash and cash equivalents	4.8	4.1	4.4	5.7	4.8		4.4	4.1	5.7
Accounts receivable, net					33.8		30.3
Inventory, net					46.0		45.3
Other current assets					0.9		0.2
Total current assets					85.5		80.2
Property, plant and equipment					179.0		175.9
Accumulated depreciation					(91.4)		(87.2)
Property, plant and equipment, net					87.6		88.7
Investment in subsidiaries					0.0		0.0
Inter-company accounts					1,504.3		1,464.0
Other non-current assets					33.0		33.0
TOTAL ASSETS					1,710.4		1,665.9
Current liabilities:
Accounts payable					15.6		16.7
Accrued liabilities					6.2		7.0
Current portion of pension and other postretirement benefit obligations					0.0		0.0
Total current liabilities					21.8		23.7
Non-current liabilities:
Long-term debt					0.0		0.0
Pension and other postretirement benefit obligations					0.0		0.0
Other non-current liabilities					1.5		1.6
TOTAL LIABILITIES					23.3		25.3
EXCHANGEABLE NOTES EXCHANGE FEATURE							0.0
Equity [Abstract]
Total stockholders' equity (deficit)					1,687.1		1,640.6
Noncontrolling interests					0.0		0.0
TOTAL EQUITY					1,687.1		1,640.6
TOTAL LIABILITIES AND EQUITY					1,710.4		1,665.9
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			19.7	23.6
Cash flows from investing activities:
Capital investments			(3.2)	(3.4)
Cash held for retirement of debt				0.0
Other investing items, net			0.0	0.0
Net cash flows from investing activities			(3.2)	(3.4)
Cash flows from financing activities:
Net borrowings (payments) under credit facility				0.0
Proceeds from issuance of long-term debt			0.0	0.0
Redemption of long-term debt			0.0	0.0
Proceeds from issuance of common stock				0.0
Debt issuance costs			0.0	0.0
Inter-company activity			(16.1)	(21.8)
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			(16.1)	(21.8)
Net increase (decrease) in cash and cash equivalents			0.4	(1.6)
Cash and cash equivalents, beginning of period			4.4	5.7
Cash and cash equivalents, end of period	4.8	4.1	4.8	4.1
Other Non-Guarantor Subsidiaries [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	97.9	104.3	189.3	219.6
Cost of products sold (exclusive of items shown separately below)	71.1	75.7	133.9	162.7
Selling and administrative expenses (exclusive of items shown separately below)	4.8	5.8	10.3	11.5
Depreciation	6.1	5.5	12.0	10.8
Pension and OPEB expense (income)	0.0	0.0	0.0	0.0
Total operating costs	82.0	87.0	156.2	185.0
Operating profit (loss)	15.9	17.3	33.1	34.6
Interest expense	0.3	0.3	0.7	0.8
Other income (expense)	1.4	5.2	2.6	6.6
Income (loss) before income taxes	17.0	22.2	35.0	40.4
Income tax expense (benefit)	0.6	2.2	1.3	2.3
Equity in net income (loss) of subsidiaries	(0.2)	0.0	(0.1)	0.1
Net income (loss)	16.2	20.0	33.6	38.2
Less: Net income attributable to noncontrolling interests	15.2	16.6	31.4	34.6
Net income (loss) attributable to AK Steel Holding Corporation	1.0	3.4	2.2	3.6
Other comprehensive income (loss)	2.3	(1.1)	2.6	0.4
Comprehensive income (loss) attributable to AK Steel Holding Corporation	3.3	2.3	4.8	4.0
Current assets:
Cash and cash equivalents	19.9	31.8	20.9	23.9	19.9		20.9	31.8	23.9
Accounts receivable, net					23.9		19.6
Inventory, net					45.7		42.0
Other current assets					2.9		2.9
Total current assets					92.4		85.4
Property, plant and equipment					536.7		535.2
Accumulated depreciation					(113.1)		(101.6)
Property, plant and equipment, net					423.6		433.6
Investment in subsidiaries					67.4		67.5
Inter-company accounts					(455.3)		(458.4)
Other non-current assets					41.9		38.9
TOTAL ASSETS					170.0		167.0
Current liabilities:
Accounts payable					21.7		15.0
Accrued liabilities					13.0		13.4
Current portion of pension and other postretirement benefit obligations					0.3		0.3
Total current liabilities					35.0		28.7
Non-current liabilities:
Long-term debt					0.0		0.0
Pension and other postretirement benefit obligations					3.5		3.3
Other non-current liabilities					0.2		0.1
TOTAL LIABILITIES					38.7		32.1
EXCHANGEABLE NOTES EXCHANGE FEATURE							0.0
Equity [Abstract]
Total stockholders' equity (deficit)					(221.1)		(228.0)
Noncontrolling interests					352.4		362.9
TOTAL EQUITY					131.3		134.9
TOTAL LIABILITIES AND EQUITY					170.0		167.0
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			43.4	52.1
Cash flows from investing activities:
Capital investments			(1.0)	(1.5)
Cash held for retirement of debt				0.0
Other investing items, net			(0.1)	2.0
Net cash flows from investing activities			(1.1)	0.5
Cash flows from financing activities:
Net borrowings (payments) under credit facility				0.0
Proceeds from issuance of long-term debt			0.0	0.0
Redemption of long-term debt			0.0	0.0
Proceeds from issuance of common stock				0.0
Debt issuance costs			0.0	0.0
Inter-company activity			(1.4)	(7.7)
SunCoke Middletown distributions to noncontrolling interest owners			(41.9)	(37.0)
Other financing items, net			0.0	0.0
Net cash flows from financing activities			(43.3)	(44.7)
Net increase (decrease) in cash and cash equivalents			(1.0)	7.9
Cash and cash equivalents, beginning of period			20.9	23.9
Cash and cash equivalents, end of period	19.9	31.8	19.9	31.8
Consolidation, Eliminations [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	(113.2)	(113.3)	(217.1)	(239.5)
Cost of products sold (exclusive of items shown separately below)	(102.3)	(101.8)	(192.7)	(215.1)
Selling and administrative expenses (exclusive of items shown separately below)	(11.7)	(10.9)	(22.9)	(22.3)
Depreciation	0.0	0.0	0.0	0.0
Pension and OPEB expense (income)	0.0	0.0	0.0	0.0
Total operating costs	(114.0)	(112.7)	(215.6)	(237.4)
Operating profit (loss)	0.8	(0.6)	(1.5)	(2.1)
Interest 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	0.8	(0.6)	(1.5)	(2.1)
Income tax expense (benefit)	0.3	(0.2)	(0.6)	(0.8)
Equity in net income (loss) of subsidiaries	(75.1)	(31.4)	(149.7)	(29.3)
Net income (loss)	(74.6)	(31.8)	(150.6)	(30.6)
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income (loss) attributable to AK Steel Holding Corporation	(74.6)	(31.8)	(150.6)	(30.6)
Other comprehensive income (loss)	17.6	(41.3)	40.0	(41.8)
Comprehensive income (loss) attributable to AK Steel Holding Corporation	(57.0)	(73.1)	(110.6)	(72.4)
Current assets:
Cash and cash equivalents	0.0	0.0	0.0	0.0	0.0		0.0	$ 0.0	$ 0.0
Accounts receivable, net					(12.0)		(8.3)
Inventory, net					(13.1)		(11.5)
Other current assets					0.0		0.0
Total current assets					(25.1)		(19.8)
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					1,660.8		1,753.4
Inter-company accounts					81.4		81.4
Other non-current assets					0.0		0.0
TOTAL ASSETS					1,717.1		1,815.0
Current liabilities:
Accounts payable					(3.1)		0.5
Accrued liabilities					0.0		0.0
Current portion of pension and other postretirement benefit obligations					0.0		0.0
Total current liabilities					(3.1)		0.5
Non-current liabilities:
Long-term debt					0.0		0.0
Pension and other postretirement benefit obligations					0.0		0.0
Other non-current liabilities					0.0		0.0
TOTAL LIABILITIES					(3.1)		0.5
EXCHANGEABLE NOTES EXCHANGE FEATURE							0.0
Equity [Abstract]
Total stockholders' equity (deficit)					1,720.2		1,814.5
Noncontrolling interests					0.0		0.0
TOTAL EQUITY					1,720.2		1,814.5
TOTAL LIABILITIES AND EQUITY					$ 1,717.1		$ 1,815.0
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			0.8	(10.5)
Cash flows from investing activities:
Capital investments			0.0	0.0
Cash held for retirement of debt				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	0.0
Redemption of long-term debt			0.0	0.0
Proceeds from issuance of common stock				0.0
Debt issuance costs			0.0	0.0
Inter-company activity			(0.8)	10.5
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			(0.8)	10.5
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
Senior Secured Notes Due July 2023 [Member]
Condensed Financial Statements, Captions [Line Items]
Debt Instrument, Interest Rate, Stated Percentage					7.50%		7.50%
Senior Notes Due October 2021 [Member]
Condensed Financial Statements, Captions [Line Items]
Debt Instrument, Interest Rate, Stated Percentage					7.625%		7.625%
Senior Notes Due April 2022 [Member]
Condensed Financial Statements, Captions [Line Items]
Debt Instrument, Interest Rate, Stated Percentage					8.375%		8.375%
Senior Notes Due March 2027 [Member]
Condensed Financial Statements, Captions [Line Items]
Debt Instrument, Interest Rate, Stated Percentage						7.00%
Cash flows from financing activities:
Debt issuance costs			$ (7.5)