Supplementary Guarantor Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	$ 1,746.6	$ 1,557.2	$ 3,405.5	$ 3,090.6
Cost of products sold (exclusive of item shown separately below)	1,512.7	1,320.0	2,976.4	2,596.9
Selling and administrative expenses	79.5	65.6	156.2	137.3
Depreciation	54.9	55.1	109.8	110.2
Total operating costs	1,647.1	1,440.7	3,242.4	2,844.4
Operating profit (loss)	99.5	116.5	163.1	246.2
Interest expense	37.9	38.2	75.5	77.6
Pension and OPEB (income) expense	(10.0)	(18.1)	(20.0)	(36.2)
Other (income) expense	(0.2)	3.6	(4.1)	11.4
Income (loss) before income taxes	71.8	92.8	111.7	193.4
Income tax expense (benefit)	(0.5)	0.4	(5.4)	0.4
Equity in net income (loss) of subsidiaries	0.0	0.0	0.0	0.0
Net income	72.3	92.4	117.1	193.0
Less: Net income attributable to noncontrolling interests	15.7	15.2	31.8	31.4
Net income attributable to AK Steel Holding Corporation	56.6	77.2	85.3	161.6
Other comprehensive income (loss)	(10.7)	(19.1)	(17.4)	(39.2)
Comprehensive income attributable to AK Steel Holding Corporation	45.9	58.1	67.9	122.4
Current assets:
Cash and Cash Equivalents, at Carrying Value	46.0				$ 38.0
Accounts receivable, net	653.0				517.8
Inventory	1,311.0				1,385.0
Other current assets	98.4				130.3
Total current assets	2,108.4				2,071.1
Property, plant and equipment	6,884.8				6,831.8
Accumulated depreciation	(4,953.6)				(4,845.6)
Property, plant and equipment, net	1,931.2				1,986.2
Investment in subsidiaries	0.0				0.0
Inter-company accounts	0.0				0.0
Other non-current assets	405.7				417.5
TOTAL ASSETS	4,445.3				4,474.8
Current liabilities:
Accounts payable	768.3				690.4
Accrued liabilities	243.4				270.5
Current portion of pension and other postretirement benefit obligations	39.2				40.1
Total current liabilities	1,050.9				1,001.0
Non-current liabilities:
Long-term debt	2,036.6				2,110.1
Pension and other postretirement benefit obligations	841.4				894.2
Inter-company accounts	0.0				0.0
Other non-current liabilities	145.6				168.9
TOTAL LIABILITIES	4,074.5				4,174.2
Equity [Abstract]
Total stockholders' equity (deficit)	28.6				(44.6)
Noncontrolling interests	342.2				345.2
TOTAL EQUITY	370.8	286.0			300.6	$ 149.8
TOTAL LIABILITIES AND EQUITY	4,445.3				4,474.8
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			187.4	203.2
Cash flows from investing activities:
Capital investments			(64.0)	(52.1)
Other investing items, net			0.3	2.9
Net cash flows from investing activities			(63.7)	(49.2)
Cash flows from financing activities:
Net borrowings (payments) under credit facility			(80.0)	0.0
Proceeds from issuance of long-term debt			0.0	400.0
Redemption of long-term debt			0.0	(538.2)
Debt issuance costs			0.0	(8.3)
Inter-company activity			0.0	0.0
SunCoke Middletown distributions to noncontrolling interest owners			(34.8)	(41.9)
Other financing items, net			(0.9)	(2.5)
Net cash flows from financing activities			(115.7)	(190.9)
Net increase (decrease) in cash and cash equivalents			8.0	(36.9)
Cash and cash equivalents, beginning of period			38.0	173.2
Cash and equivalents, end of period	46.0	136.3	38.0	173.2	38.0	173.2
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 item shown separately below)	0.0	0.0	0.0	0.0
Selling and administrative expenses	0.8	1.0	2.1	2.4
Depreciation	0.0	0.0	0.0	0.0
Total operating costs	0.8	1.0	2.1	2.4
Operating profit (loss)	(0.8)	(1.0)	(2.1)	(2.4)
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	(0.8)	(1.0)	(2.1)	(2.4)
Income tax expense (benefit)	0.0	0.0	0.0	0.0
Equity in net income (loss) of subsidiaries	57.4	78.2	87.4	164.0
Net income	56.6	77.2	85.3	161.6
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income attributable to AK Steel Holding Corporation	56.6	77.2	85.3	161.6
Other comprehensive income (loss)	(10.7)	(19.1)	(17.4)	(39.2)
Comprehensive income attributable to AK Steel Holding Corporation	45.9	58.1	67.9	122.4
Current assets:
Cash and Cash Equivalents, at Carrying Value	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	(3,122.3)				(3,127.4)
Inter-company accounts	3,150.9				3,082.8
Other non-current assets	0.0				0.0
TOTAL ASSETS	28.6				(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)	28.6				(44.6)
Noncontrolling interests	0.0				0.0
TOTAL EQUITY	28.6				(44.6)
TOTAL LIABILITIES AND EQUITY	28.6				(44.6)
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			(1.5)	(1.8)
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			2.4	4.2
SunCoke Middletown distributions to noncontrolling interest owners			0.0	0.0
Other financing items, net			(0.9)	(2.4)
Net cash flows from financing activities			1.5	1.8
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 equivalents, end of period	0.0	0.0	0.0	0.0	0.0	0.0
AK Steel [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	1,599.3	1,498.4	3,102.3	2,972.0
Cost of products sold (exclusive of item shown separately below)	1,413.4	1,298.0	2,766.8	2,556.0
Selling and administrative expenses	78.7	68.3	153.6	141.1
Depreciation	43.4	46.9	86.8	94.1
Total operating costs	1,535.5	1,413.2	3,007.2	2,791.2
Operating profit (loss)	63.8	85.2	95.1	180.8
Interest expense	36.5	37.9	72.9	76.9
Pension and OPEB (income) expense	(10.0)	(18.1)	(20.0)	(36.2)
Other (income) expense	4.6	7.8	5.8	19.2
Income (loss) before income taxes	32.7	57.6	36.4	120.9
Income tax expense (benefit)	(6.0)	(7.5)	(17.9)	(16.1)
Equity in net income (loss) of subsidiaries	18.7	13.1	33.1	27.0
Net income	57.4	78.2	87.4	164.0
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income attributable to AK Steel Holding Corporation	57.4	78.2	87.4	164.0
Other comprehensive income (loss)	(10.7)	(19.1)	(17.4)	(39.2)
Comprehensive income attributable to AK Steel Holding Corporation	46.7	59.1	70.0	124.8
Current assets:
Cash and Cash Equivalents, at Carrying Value	27.0				14.5
Accounts receivable, net	526.8				432.4
Inventory	1,184.0				1,231.5
Other current assets	90.5				124.7
Total current assets	1,828.3				1,803.1
Property, plant and equipment	6,048.1				6,004.0
Accumulated depreciation	(4,706.5)				(4,620.8)
Property, plant and equipment, net	1,341.6				1,383.2
Investment in subsidiaries	1,902.6				1,844.9
Inter-company accounts	0.0				0.0
Other non-current assets	55.7				66.3
TOTAL ASSETS	5,128.2				5,097.5
Current liabilities:
Accounts payable	693.4				623.3
Accrued liabilities	206.6				228.2
Current portion of pension and other postretirement benefit obligations	38.9				39.7
Total current liabilities	938.9				891.2
Non-current liabilities:
Long-term debt	2,036.6				2,110.1
Pension and other postretirement benefit obligations	838.2				890.8
Inter-company accounts	4,319.1				4,199.6
Other non-current liabilities	117.7				133.2
TOTAL LIABILITIES	8,250.5				8,224.9
Equity [Abstract]
Total stockholders' equity (deficit)	(3,122.3)				(3,127.4)
Noncontrolling interests	0.0				0.0
TOTAL EQUITY	(3,122.3)				(3,127.4)
TOTAL LIABILITIES AND EQUITY	5,128.2				5,097.5
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			123.0	141.1
Cash flows from investing activities:
Capital investments			(51.8)	(47.9)
Other investing items, net			(1.0)	3.0
Net cash flows from investing activities			(52.8)	(44.9)
Cash flows from financing activities:
Net borrowings (payments) under credit facility			(80.0)
Proceeds from issuance of long-term debt				400.0
Redemption of long-term debt				(538.2)
Debt issuance costs				(8.3)
Inter-company activity			22.3	14.1
SunCoke Middletown distributions to noncontrolling interest owners			0.0	0.0
Other financing items, net			0.0	(0.1)
Net cash flows from financing activities			(57.7)	(132.5)
Net increase (decrease) in cash and cash equivalents			12.5	(36.3)
Cash and cash equivalents, beginning of period			14.5	147.9
Cash and equivalents, end of period	27.0	111.6	14.5	147.9	14.5	147.9
Guarantor Subsidiaries [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	84.4	74.1	161.5	146.4
Cost of products sold (exclusive of item shown separately below)	57.4	53.2	112.0	99.7
Selling and administrative expenses	3.8	3.2	7.4	6.4
Depreciation	2.0	2.1	4.0	4.1
Total operating costs	63.2	58.5	123.4	110.2
Operating profit (loss)	21.2	15.6	38.1	36.2
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	(4.0)	(2.8)	(7.5)	(5.2)
Income (loss) before income taxes	25.2	18.4	45.6	41.4
Income tax expense (benefit)	6.3	7.0	11.4	15.8
Equity in net income (loss) of subsidiaries	0.0	0.0	0.0	0.0
Net income	18.9	11.4	34.2	25.6
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income attributable to AK Steel Holding Corporation	18.9	11.4	34.2	25.6
Other comprehensive income (loss)	0.0	0.0	0.0	0.0
Comprehensive income attributable to AK Steel Holding Corporation	18.9	11.4	34.2	25.6
Current assets:
Cash and Cash Equivalents, at Carrying Value	5.1				7.2
Accounts receivable, net	40.5				33.2
Inventory	55.7				55.1
Other current assets	0.2				0.2
Total current assets	101.5				95.7
Property, plant and equipment	183.5				181.5
Accumulated depreciation	(98.7)				(94.8)
Property, plant and equipment, net	84.8				86.7
Investment in subsidiaries	0.0				0.0
Inter-company accounts	1,587.9				1,523.1
Other non-current assets	32.9				33.0
TOTAL ASSETS	1,807.1				1,738.5
Current liabilities:
Accounts payable	24.2				17.9
Accrued liabilities	7.0				7.8
Current portion of pension and other postretirement benefit obligations	0.0				0.0
Total current liabilities	31.2				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	31.2				26.7
Equity [Abstract]
Total stockholders' equity (deficit)	1,775.9				1,711.8
Noncontrolling interests	0.0				0.0
TOTAL EQUITY	1,775.9				1,711.8
TOTAL LIABILITIES AND EQUITY	1,807.1				1,738.5
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			31.1	19.7
Cash flows from investing activities:
Capital investments			(2.1)	(3.2)
Other investing items, net			0.0	0.0
Net cash flows from investing activities			(2.1)	(3.2)
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			(31.1)	(16.1)
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			(31.1)	(16.1)
Net increase (decrease) in cash and cash equivalents			(2.1)	0.4
Cash and cash equivalents, beginning of period			7.2	4.4
Cash and equivalents, end of period	5.1	4.8	7.2	4.4	7.2	4.4
Other Non-Guarantor Subsidiaries [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	195.1	97.9	403.8	189.3
Cost of products sold (exclusive of item shown separately below)	159.7	71.1	331.4	133.9
Selling and administrative expenses	9.0	4.8	17.7	10.3
Depreciation	9.5	6.1	19.0	12.0
Total operating costs	178.2	82.0	368.1	156.2
Operating profit (loss)	16.9	15.9	35.7	33.1
Interest expense	1.4	0.3	2.6	0.7
Pension and OPEB (income) expense	0.0	0.0	0.0	0.0
Other (income) expense	(0.8)	(1.4)	(2.4)	(2.6)
Income (loss) before income taxes	16.3	17.0	35.5	35.0
Income tax expense (benefit)	(0.4)	0.6	2.0	1.3
Equity in net income (loss) of subsidiaries	0.2	(0.2)	0.2	(0.1)
Net income	16.9	16.2	33.7	33.6
Less: Net income attributable to noncontrolling interests	15.7	15.2	31.8	31.4
Net income attributable to AK Steel Holding Corporation	1.2	1.0	1.9	2.2
Other comprehensive income (loss)	(2.2)	2.3	(1.1)	2.6
Comprehensive income attributable to AK Steel Holding Corporation	(1.0)	3.3	0.8	4.8
Current assets:
Cash and Cash Equivalents, at Carrying Value	13.9				16.3
Accounts receivable, net	96.0				65.4
Inventory	85.0				108.3
Other current assets	7.7				5.4
Total current assets	202.6				195.4
Property, plant and equipment	653.2				646.3
Accumulated depreciation	(148.4)				(130.0)
Property, plant and equipment, net	504.8				516.3
Investment in subsidiaries	67.7				67.5
Inter-company accounts	0.0				0.0
Other non-current assets	317.1				318.2
TOTAL ASSETS	1,092.2				1,097.4
Current liabilities:
Accounts payable	53.8				50.9
Accrued liabilities	29.8				34.5
Current portion of pension and other postretirement benefit obligations	0.3				0.4
Total current liabilities	83.9				85.8
Non-current liabilities:
Long-term debt	0.0				0.0
Pension and other postretirement benefit obligations	3.2				3.4
Inter-company accounts	499.6				489.1
Other non-current liabilities	27.9				34.7
TOTAL LIABILITIES	614.6				613.0
Equity [Abstract]
Total stockholders' equity (deficit)	135.4				139.2
Noncontrolling interests	342.2				345.2
TOTAL EQUITY	477.6				484.4
TOTAL LIABILITIES AND EQUITY	1,092.2				1,097.4
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			38.1	43.4
Cash flows from investing activities:
Capital investments			(10.1)	(1.0)
Other investing items, net			1.3	(0.1)
Net cash flows from investing activities			(8.8)	(1.1)
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			3.1	(1.4)
SunCoke Middletown distributions to noncontrolling interest owners			(34.8)	(41.9)
Other financing items, net			0.0	0.0
Net cash flows from financing activities			(31.7)	(43.3)
Net increase (decrease) in cash and cash equivalents			(2.4)	(1.0)
Cash and cash equivalents, beginning of period			16.3	20.9
Cash and equivalents, end of period	13.9	19.9	16.3	20.9	16.3	20.9
Consolidation, Eliminations [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	(132.2)	(113.2)	(262.1)	(217.1)
Cost of products sold (exclusive of item shown separately below)	(117.8)	(102.3)	(233.8)	(192.7)
Selling and administrative expenses	(12.8)	(11.7)	(24.6)	(22.9)
Depreciation	0.0	0.0	0.0	0.0
Total operating costs	(130.6)	(114.0)	(258.4)	(215.6)
Operating profit (loss)	(1.6)	0.8	(3.7)	(1.5)
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	(1.6)	0.8	(3.7)	(1.5)
Income tax expense (benefit)	(0.4)	0.3	(0.9)	(0.6)
Equity in net income (loss) of subsidiaries	(76.3)	(91.1)	(120.7)	(190.9)
Net income	(77.5)	(90.6)	(123.5)	(191.8)
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income attributable to AK Steel Holding Corporation	(77.5)	(90.6)	(123.5)	(191.8)
Other comprehensive income (loss)	12.9	16.8	18.5	36.6
Comprehensive income attributable to AK Steel Holding Corporation	(64.6)	(73.8)	(105.0)	(155.2)
Current assets:
Cash and Cash Equivalents, at Carrying Value	0.0				0.0
Accounts receivable, net	(10.3)				(13.2)
Inventory	(13.7)				(9.9)
Other current assets	0.0				0.0
Total current assets	(24.0)				(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	1,152.0				1,215.0
Inter-company accounts	(4,738.8)				(4,605.9)
Other non-current assets	0.0				0.0
TOTAL ASSETS	(3,610.8)				(3,414.0)
Current liabilities:
Accounts payable	(3.1)				(1.7)
Accrued liabilities	0.0				0.0
Current portion of pension and other postretirement benefit obligations	0.0				0.0
Total current liabilities	(3.1)				(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	(4,818.7)				(4,688.7)
Other non-current liabilities	0.0				0.0
TOTAL LIABILITIES	(4,821.8)				(4,690.4)
Equity [Abstract]
Total stockholders' equity (deficit)	1,211.0				1,276.4
Noncontrolling interests	0.0				0.0
TOTAL EQUITY	1,211.0				1,276.4
TOTAL LIABILITIES AND EQUITY	(3,610.8)				(3,414.0)
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			(3.3)	0.8
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			3.3	(0.8)
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			3.3	(0.8)
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 equivalents, end of period	$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 0.0