Supplemental Guarantor Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	$ 1,689.4	$ 1,530.8	$ 3,440.3	$ 2,914.3
Cost of products sold (exclusive of items shown separately below)	1,579.2	1,416.9	3,187.8	2,752.5
Selling and administrative expenses (exclusive of items shown separately below)	63.5	53.9	132.7	114.1
Depreciation	55.7	48.5	111.1	97.2
Pension and OPEB expense (income)	(16.1)	(25.0)	(32.2)	(50.7)
Total operating costs	1,682.3	1,494.3	3,399.4	2,913.1
Operating profit (loss)	7.1	36.5	40.9	1.2
Interest expense	43.5	33.2	87.4	65.4
Impairment of Magnetation investment	0.0	0.0	(256.3)	0.0
Other income (expense)	1.5	(3.0)	(15.2)	(4.9)
Income (loss) before income taxes	(34.9)	0.3	(318.0)	(69.1)
Income tax expense (benefit)	14.6	1.8	22.3	3.6
Equity in net income (loss) of subsidiaries	0.0	0.0	0.0	0.0
Net income (loss)	(49.5)	(1.5)	(340.3)	(72.7)
Less: Net income attributable to noncontrolling interests	14.5	15.6	30.0	30.5
Net income (loss) attributable to AK Steel Holding Corporation	(64.0)	(17.1)	(370.3)	(103.2)
Other comprehensive income (loss)	2.2	(27.2)	(8.0)	(51.4)
Comprehensive income (loss) attributable to AK Steel Holding Corporation	(61.8)	(44.3)	(378.3)	(154.6)
Current assets:
Cash and cash equivalents	75.0	54.8	70.2	45.3	$ 70.2	$ 45.3
Accounts receivable, net	567.5				644.3
Inventory, net	1,156.5				1,172.1
Deferred tax assets, current	57.5				67.7
Other current assets	57.8				71.4
Total current assets	1,914.3				2,025.7
Property, plant and equipment	6,419.1				6,388.4
Accumulated depreciation	(4,283.6)				(4,175.2)
Property, plant and equipment, net	2,135.5				2,213.2
Other non-current assets:
Investments in affilates	76.3				388.7
Investment in subsidiaries	0.0				0.0
Inter-company accounts	0.0				0.0
Other non-current assets	209.3				230.9
TOTAL ASSETS	4,335.4				4,858.5
Current liabilities:
Accounts payable	761.8				803.1
Accrued liabilities	225.4				266.5
Current portion of pension and other postretirement benefit obligations	63.7				55.6
Total current liabilities	1,050.9				1,125.2
Non-current liabilities:
Long-term debt	2,442.5				2,452.5
Pension and other postretirement benefit obligations	1,174.2				1,225.3
Other non-current liabilities	130.8				132.5
TOTAL LIABILITIES	4,798.4				4,935.5
Total stockholders' equity (deficit)	(865.9)				(492.5)
Noncontrolling interests	402.9				415.5
TOTAL EQUITY (DEFICIT)	(463.0)	44.3			(77.0)	192.7
TOTAL LIABILITIES AND EQUITY (DEFICIT)	4,335.4				4,858.5
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			83.9	(331.1)
Cash flows from investing activities:
Capital investments			(49.7)	(27.6)
Investments in Magnetation joint venture			0.0	(45.0)
Proceeds from sale of equity investee			25.0	0.0
Other investing items, net			1.3	6.9
Net cash flows from investing activities			(23.4)	(65.7)
Cash flows from financing activities:
Net borrowings (payments) under credit facility			(10.0)	440.0
Redemption of long-term debt			(2.2)	(0.4)
Debt issuance costs			0.0	(3.3)
Inter-company activity			0.0	0.0
SunCoke Middletown distributions to noncontrolling interest owners			(42.6)	(28.9)
Other financing items, net			(0.9)	(1.1)
Net cash flows from financing activities			(55.7)	406.3
Net increase (decrease) in cash and cash equivalents			4.8	9.5
Cash and cash equivalents, beginning of period			70.2	45.3
Cash and cash equivalents, end of period	75.0	54.8	75.0	54.8
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)	1.4	1.0	3.1	2.6
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	1.4	1.0	3.1	2.6
Operating profit (loss)	(1.4)	(1.0)	(3.1)	(2.6)
Interest expense	0.0	0.0	0.0	0.0
Impairment of Magnetation investment			0.0
Other income (expense)	0.0	0.0	0.0	0.0
Income (loss) before income taxes	(1.4)	(1.0)	(3.1)	(2.6)
Income tax expense (benefit)	0.0	0.0	0.0	0.0
Equity in net income (loss) of subsidiaries	(62.6)	(16.1)	(367.2)	(100.6)
Net income (loss)	(64.0)	(17.1)	(370.3)	(103.2)
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income (loss) attributable to AK Steel Holding Corporation	(64.0)	(17.1)	(370.3)	(103.2)
Other comprehensive income (loss)	2.2	(27.2)	(8.0)	(51.4)
Comprehensive income (loss) attributable to AK Steel Holding Corporation	(61.8)	(44.3)	(378.3)	(154.6)
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, net	0.0				0.0
Deferred tax assets, current	0.0				0.0
Other current assets	0.3				0.3
Total current assets	0.3				0.3
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:
Investments in affilates	0.0				0.0
Investment in subsidiaries	(3,393.3)				(2,970.9)
Inter-company accounts	2,527.1				2,478.1
Other non-current assets	0.0				0.0
TOTAL ASSETS	(865.9)				(492.5)
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
Total stockholders' equity (deficit)	(865.9)				(492.5)
Noncontrolling interests	0.0				0.0
TOTAL EQUITY (DEFICIT)	(865.9)				(492.5)
TOTAL LIABILITIES AND EQUITY (DEFICIT)	(865.9)				(492.5)
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			(2.5)	(2.0)
Cash flows from investing activities:
Capital investments			0.0	0.0
Investments in Magnetation joint venture				0.0
Proceeds from sale of equity investee			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	0.0
Debt issuance costs				0.0
Inter-company activity			3.5	3.0
SunCoke Middletown distributions to noncontrolling interest owners			0.0	0.0
Other financing items, net			(1.0)	(1.0)
Net cash flows from financing activities			2.5	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	1,639.3	1,483.1	3,341.8	2,798.2
Cost of products sold (exclusive of items shown separately below)	1,556.2	1,404.1	3,165.6	2,708.0
Selling and administrative expenses (exclusive of items shown separately below)	65.4	53.8	136.2	112.7
Depreciation	48.5	42.3	96.9	84.8
Pension and OPEB expense (income)	(16.1)	(25.0)	(32.2)	(50.7)
Total operating costs	1,654.0	1,475.2	3,366.5	2,854.8
Operating profit (loss)	(14.7)	7.9	(24.7)	(56.6)
Interest expense	43.0	32.5	86.4	64.2
Impairment of Magnetation investment			0.0
Other income (expense)	(1.6)	(3.9)	(4.2)	(7.7)
Income (loss) before income taxes	(59.3)	(28.5)	(115.3)	(128.5)
Income tax expense (benefit)	10.3	(4.7)	11.8	(9.3)
Equity in net income (loss) of subsidiaries	7.0	7.7	(240.1)	18.6
Net income (loss)	(62.6)	(16.1)	(367.2)	(100.6)
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income (loss) attributable to AK Steel Holding Corporation	(62.6)	(16.1)	(367.2)	(100.6)
Other comprehensive income (loss)	2.2	(27.2)	(8.0)	(51.4)
Comprehensive income (loss) attributable to AK Steel Holding Corporation	(60.4)	(43.3)	(375.2)	(152.0)
Current assets:
Cash and cash equivalents	27.9	19.0	28.5	16.8	28.5	16.8
Accounts receivable, net	535.6				606.2
Inventory, net	1,073.8				1,080.5
Deferred tax assets, current	57.3				67.5
Other current assets	54.1				67.9
Total current assets	1,748.7				1,850.6
Property, plant and equipment	5,721.7				5,695.8
Accumulated depreciation	(4,135.7)				(4,040.8)
Property, plant and equipment, net	1,586.0				1,655.0
Other non-current assets:
Investments in affilates	42.7				84.5
Investment in subsidiaries	1,369.5				1,582.4
Inter-company accounts	(3,518.1)				(3,420.4)
Other non-current assets	116.0				137.4
TOTAL ASSETS	1,344.8				1,889.5
Current liabilities:
Accounts payable	724.8				754.9
Accrued liabilities	206.3				244.6
Current portion of pension and other postretirement benefit obligations	63.4				55.3
Total current liabilities	994.5				1,054.8
Non-current liabilities:
Long-term debt	2,442.5				2,452.5
Pension and other postretirement benefit obligations	1,170.7				1,221.3
Other non-current liabilities	130.4				131.8
TOTAL LIABILITIES	4,738.1				4,860.4
Total stockholders' equity (deficit)	(3,393.3)				(2,970.9)
Noncontrolling interests	0.0				0.0
TOTAL EQUITY (DEFICIT)	(3,393.3)				(2,970.9)
TOTAL LIABILITIES AND EQUITY (DEFICIT)	1,344.8				1,889.5
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			14.9	(375.3)
Cash flows from investing activities:
Capital investments			(45.7)	(16.7)
Investments in Magnetation joint venture				0.0
Proceeds from sale of equity investee			25.0
Other investing items, net			1.3	7.2
Net cash flows from investing activities			(19.4)	(9.5)
Cash flows from financing activities:
Net borrowings (payments) under credit facility			(10.0)	440.0
Redemption of long-term debt			(2.2)	(0.4)
Debt issuance costs				(3.3)
Inter-company activity			16.0	(49.2)
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			3.9	387.0
Net increase (decrease) in cash and cash equivalents			(0.6)	2.2
Cash and cash equivalents, beginning of period			28.5	16.8
Cash and cash equivalents, end of period	27.9	19.0	27.9	19.0
Guarantor Subsidiaries [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	65.0	72.3	130.8	140.1
Cost of products sold (exclusive of items shown separately below)	42.2	51.9	87.0	101.5
Selling and administrative expenses (exclusive of items shown separately below)	2.7	2.9	5.7	5.7
Depreciation	1.0	1.0	2.0	2.0
Pension and OPEB expense (income)	0.0	0.0	0.0	0.0
Total operating costs	45.9	55.8	94.7	109.2
Operating profit (loss)	19.1	16.5	36.1	30.9
Interest expense	0.0	0.0	0.0	0.0
Impairment of Magnetation investment			0.0
Other income (expense)	1.6	1.7	3.2	3.3
Income (loss) before income taxes	20.7	18.2	39.3	34.2
Income tax expense (benefit)	8.2	7.9	15.7	13.7
Equity in net income (loss) of subsidiaries	0.0	0.0	0.0	0.0
Net income (loss)	12.5	10.3	23.6	20.5
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income (loss) attributable to AK Steel Holding Corporation	12.5	10.3	23.6	20.5
Other comprehensive income (loss)	0.0	0.0	0.0	0.0
Comprehensive income (loss) attributable to AK Steel Holding Corporation	12.5	10.3	23.6	20.5
Current assets:
Cash and cash equivalents	0.0	0.2	0.5	0.0	0.5	0.0
Accounts receivable, net	27.9				27.6
Inventory, net	26.8				26.2
Deferred tax assets, current	0.0				0.0
Other current assets	0.3				0.2
Total current assets	55.0				54.5
Property, plant and equipment	99.3				97.3
Accumulated depreciation	(73.8)				(71.8)
Property, plant and equipment, net	25.5				25.5
Other non-current assets:
Investments in affilates	0.0				0.0
Investment in subsidiaries	0.0				0.0
Inter-company accounts	1,376.4				1,325.5
Other non-current assets	33.0				33.0
TOTAL ASSETS	1,489.9				1,438.5
Current liabilities:
Accounts payable	5.8				6.9
Accrued liabilities	2.8				3.0
Current portion of pension and other postretirement benefit obligations	0.0				0.0
Total current liabilities	8.6				9.9
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	8.6				9.9
Total stockholders' equity (deficit)	1,481.3				1,428.6
Noncontrolling interests	0.0				0.0
TOTAL EQUITY (DEFICIT)	1,481.3				1,428.6
TOTAL LIABILITIES AND EQUITY (DEFICIT)	1,489.9				1,438.5
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			22.9	15.6
Cash flows from investing activities:
Capital investments			(1.6)	(1.3)
Investments in Magnetation joint venture				0.0
Proceeds from sale of equity investee			0.0
Other investing items, net			0.0	0.0
Net cash flows from investing activities			(1.6)	(1.3)
Cash flows from financing activities:
Net borrowings (payments) under credit facility			0.0	0.0
Redemption of long-term debt			0.0	0.0
Debt issuance costs				0.0
Inter-company activity			(21.8)	(14.1)
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			(21.8)	(14.1)
Net increase (decrease) in cash and cash equivalents			(0.5)	0.2
Cash and cash equivalents, beginning of period			0.5	0.0
Cash and cash equivalents, end of period	0.0	0.2	0.0	0.2
Other Non-Guarantor Subsidiaries [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	136.1	133.2	260.5	258.0
Cost of products sold (exclusive of items shown separately below)	108.7	105.3	203.8	204.6
Selling and administrative expenses (exclusive of items shown separately below)	6.5	7.1	13.1	14.6
Depreciation	6.2	5.2	12.2	10.4
Pension and OPEB expense (income)	0.0	0.0	0.0	0.0
Total operating costs	121.4	117.6	229.1	229.6
Operating profit (loss)	14.7	15.6	31.4	28.4
Interest expense	0.5	0.7	1.0	1.2
Impairment of Magnetation investment			(256.3)
Other income (expense)	1.5	(0.8)	(14.2)	(0.5)
Income (loss) before income taxes	15.7	14.1	(240.1)	26.7
Income tax expense (benefit)	0.5	(0.4)	(5.6)	(1.2)
Equity in net income (loss) of subsidiaries	0.0	0.0	0.0	0.0
Net income (loss)	15.2	14.5	(234.5)	27.9
Less: Net income attributable to noncontrolling interests	14.5	15.6	30.0	30.5
Net income (loss) attributable to AK Steel Holding Corporation	0.7	(1.1)	(264.5)	(2.6)
Other comprehensive income (loss)	1.2	(0.1)	(2.0)	(0.1)
Comprehensive income (loss) attributable to AK Steel Holding Corporation	1.9	(1.2)	(266.5)	(2.7)
Current assets:
Cash and cash equivalents	47.1	35.6	41.2	28.5	41.2	28.5
Accounts receivable, net	39.6				37.3
Inventory, net	62.9				73.5
Deferred tax assets, current	0.2				0.2
Other current assets	3.1				3.0
Total current assets	152.9				155.2
Property, plant and equipment	598.1				595.3
Accumulated depreciation	(74.1)				(62.6)
Property, plant and equipment, net	524.0				532.7
Other non-current assets:
Investments in affilates	33.6				304.2
Investment in subsidiaries	0.0				0.0
Inter-company accounts	(422.7)				(412.4)
Other non-current assets	60.3				60.5
TOTAL ASSETS	348.1				640.2
Current liabilities:
Accounts payable	32.2				42.0
Accrued liabilities	16.3				18.9
Current portion of pension and other postretirement benefit obligations	0.3				0.3
Total current liabilities	48.8				61.2
Non-current liabilities:
Long-term debt	0.0				0.0
Pension and other postretirement benefit obligations	3.5				4.0
Other non-current liabilities	0.4				0.7
TOTAL LIABILITIES	52.7				65.9
Total stockholders' equity (deficit)	(107.5)				158.8
Noncontrolling interests	402.9				415.5
TOTAL EQUITY (DEFICIT)	295.4				574.3
TOTAL LIABILITIES AND EQUITY (DEFICIT)	348.1				640.2
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			40.4	26.3
Cash flows from investing activities:
Capital investments			(2.4)	(9.6)
Investments in Magnetation joint venture				(45.0)
Proceeds from sale of equity investee			0.0
Other investing items, net			0.0	(0.3)
Net cash flows from investing activities			(2.4)	(54.9)
Cash flows from financing activities:
Net borrowings (payments) under credit facility			0.0	0.0
Redemption of long-term debt			0.0	0.0
Debt issuance costs				0.0
Inter-company activity			10.5	64.6
SunCoke Middletown distributions to noncontrolling interest owners			(42.6)	(28.9)
Other financing items, net			0.0	0.0
Net cash flows from financing activities			(32.1)	35.7
Net increase (decrease) in cash and cash equivalents			5.9	7.1
Cash and cash equivalents, beginning of period			41.2	28.5
Cash and cash equivalents, end of period	47.1	35.6	47.1	35.6
Eliminations [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	(151.0)	(157.8)	(292.8)	(282.0)
Cost of products sold (exclusive of items shown separately below)	(127.9)	(144.4)	(268.6)	(261.6)
Selling and administrative expenses (exclusive of items shown separately below)	(12.5)	(10.9)	(25.4)	(21.5)
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	(140.4)	(155.3)	(294.0)	(283.1)
Operating profit (loss)	(10.6)	(2.5)	1.2	1.1
Interest expense	0.0	0.0	0.0	0.0
Impairment of Magnetation investment			0.0
Other income (expense)	0.0	0.0	0.0	0.0
Income (loss) before income taxes	(10.6)	(2.5)	1.2	1.1
Income tax expense (benefit)	(4.4)	(1.0)	0.4	0.4
Equity in net income (loss) of subsidiaries	55.6	8.4	607.3	82.0
Net income (loss)	49.4	6.9	608.1	82.7
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income (loss) attributable to AK Steel Holding Corporation	49.4	6.9	608.1	82.7
Other comprehensive income (loss)	(3.4)	27.3	10.0	51.5
Comprehensive income (loss) attributable to AK Steel Holding Corporation	46.0	34.2	618.1	134.2
Current assets:
Cash and cash equivalents	0.0	0.0	0.0	0.0	0.0	$ 0.0
Accounts receivable, net	(35.6)				(26.8)
Inventory, net	(7.0)				(8.1)
Deferred tax assets, current	0.0				0.0
Other current assets	0.0				0.0
Total current assets	(42.6)				(34.9)
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:
Investments in affilates	0.0				0.0
Investment in subsidiaries	2,023.8				1,388.5
Inter-company accounts	37.3				29.2
Other non-current assets	0.0				0.0
TOTAL ASSETS	2,018.5				1,382.8
Current liabilities:
Accounts payable	(1.0)				(0.7)
Accrued liabilities	0.0				0.0
Current portion of pension and other postretirement benefit obligations	0.0				0.0
Total current liabilities	(1.0)				(0.7)
Non-current liabilities:
Long-term debt	0.0				0.0
Pension and other postretirement benefit obligations	0.0				0.0
Other non-current liabilities	0.0				0.0
TOTAL LIABILITIES	(1.0)				(0.7)
Total stockholders' equity (deficit)	2,019.5				1,383.5
Noncontrolling interests	0.0				0.0
TOTAL EQUITY (DEFICIT)	2,019.5				1,383.5
TOTAL LIABILITIES AND EQUITY (DEFICIT)	2,018.5				$ 1,382.8
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities			8.2	4.3
Cash flows from investing activities:
Capital investments			0.0	0.0
Investments in Magnetation joint venture				0.0
Proceeds from sale of equity investee			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	0.0
Debt issuance costs				0.0
Inter-company activity			(8.2)	(4.3)
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			(8.2)	(4.3)
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 December 2018 [Member]
Condensed Financial Statements, Captions [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	8.75%				8.75%
Senior Notes Due May 2020 [Member]
Condensed Financial Statements, Captions [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.625%				7.625%
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%
Exchangeable Senior Notes Due November 2019 [Member]
Condensed Financial Statements, Captions [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.00%				5.00%