Supplemental Guarantor Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	$ 1,518.8	$ 1,750.9
Cost of products sold (exclusive of items shown separately below)	1,365.5	1,608.6
Selling and administrative expenses (exclusive of items shown separately below)	63.5	69.2
Depreciation	53.7	55.4
Pension and OPEB expense (income)	(11.9)	(16.1)
Total operating costs	1,470.8	1,717.1
Operating profit (loss)	48.0	33.8
Interest expense	42.8	43.9
Impairment of Magnetation investment	0.0	(256.3)
Other income (expense)	(0.7)	(16.7)
Income (loss) before income taxes	4.5	(283.1)
Income tax expense (benefit)	0.1	7.7
Equity in net income (loss) of subsidiaries	0.0	0.0
Net income (loss)	4.4	(290.8)
Less: Net income attributable to noncontrolling interests	18.0	15.5
Net income (loss) attributable to AK Steel Holding Corporation	(13.6)	(306.3)
Other comprehensive income (loss)	(1.0)	(10.2)
Comprehensive income (loss) attributable to AK Steel Holding Corporation	(14.6)	(316.5)
Current assets:
Cash and cash equivalents	56.6	70.2	$ 113.0	$ 56.6	$ 89.4	$ 70.2
Accounts receivable, net			488.7	444.9
Inventory, net			1,068.9	1,226.3
Other current assets			71.8	78.4
Total current assets			1,742.4	1,806.2
Property, plant and equipment			6,488.4	6,466.0
Accumulated depreciation			(4,432.7)	(4,379.5)
Property, plant and equipment, net			2,055.7	2,086.5
Investment in subsidiaries			0.0	0.0
Inter-company accounts			0.0	0.0
Other non-current assets			189.2	191.7
TOTAL ASSETS			3,987.3	4,084.4
Current liabilities:
Accounts payable			643.3	703.4
Accrued liabilities			270.0	261.5
Current portion of pension and other postretirement benefit obligations			78.4	77.7
Total current liabilities			991.7	1,042.6
Non-current liabilities:
Long-term debt			2,336.4	2,354.1
Pension and other postretirement benefit obligations			1,132.4	1,146.9
Other non-current liabilities			138.4	136.4
TOTAL LIABILITIES			4,598.9	4,680.0
Equity [Abstract]
Total stockholders' equity (deficit)			(990.5)	(977.6)
Noncontrolling interests			378.9	382.0
TOTAL EQUITY (DEFICIT)			(611.6)	(595.6)	(392.9)	(77.0)
TOTAL LIABILITIES AND EQUITY (DEFICIT)			3,987.3	4,084.4
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities	136.7	(2.7)
Cash flows from investing activities:
Capital investments	(28.8)	(28.3)
Other investing items, net	(0.1)	(5.6)
Net cash flows from investing activities	(28.9)	(33.9)
Cash flows from financing activities:
Net borrowings (payments) under credit facility	(30.0)	75.0
Inter-company activity	0.0	0.0
SunCoke Middletown distributions to noncontrolling interest owners	(21.1)	(18.3)
Other financing items, net	(0.3)	(0.9)
Net cash flows from financing activities	(51.4)	55.8
Net increase (decrease) in cash and cash equivalents	56.4	19.2
Cash and cash equivalents, beginning of period	56.6	70.2
Cash and cash equivalents, end of period	113.0	89.4
AK Holding [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	0.0	0.0
Cost of products sold (exclusive of items shown separately below)	0.0	0.0
Selling and administrative expenses (exclusive of items shown separately below)	1.3	1.7
Depreciation	0.0	0.0
Pension and OPEB expense (income)	0.0	0.0
Total operating costs	1.3	1.7
Operating profit (loss)	(1.3)	(1.7)
Interest expense	0.0	0.0
Impairment of Magnetation investment		0.0
Other income (expense)	0.0	0.0
Income (loss) before income taxes	(1.3)	(1.7)
Income tax expense (benefit)	0.0	0.0
Equity in net income (loss) of subsidiaries	(11.1)	(304.5)
Net income (loss)	(12.4)	(306.2)
Less: Net income attributable to noncontrolling interests	0.0	0.0
Net income (loss) attributable to AK Steel Holding Corporation	(12.4)	(306.2)
Other comprehensive income (loss)	(1.0)	(10.2)
Comprehensive income (loss) attributable to AK Steel Holding Corporation	(13.4)	(316.4)
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
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			(3,311.1)	(3,541.0)
Inter-company accounts			2,320.6	2,563.4
Other non-current assets			0.0	0.0
TOTAL ASSETS			(990.5)	(977.6)
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
Other non-current liabilities			0.0	0.0
TOTAL LIABILITIES			0.0	0.0
Equity [Abstract]
Total stockholders' equity (deficit)			(990.5)	(977.6)
Noncontrolling interests			0.0	0.0
TOTAL EQUITY (DEFICIT)			(990.5)	(977.6)
TOTAL LIABILITIES AND EQUITY (DEFICIT)			(990.5)	(977.6)
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities	(1.3)	(1.4)
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
Inter-company activity	1.6	2.3
SunCoke Middletown distributions to noncontrolling interest owners	0.0	0.0
Other financing items, net	(0.3)	(0.9)
Net cash flows from financing activities	1.3	1.4
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
AK Steel [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	1,468.2	1,702.5
Cost of products sold (exclusive of items shown separately below)	1,351.3	1,609.4
Selling and administrative expenses (exclusive of items shown separately below)	64.5	70.8
Depreciation	46.4	48.4
Pension and OPEB expense (income)	(11.9)	(16.1)
Total operating costs	1,450.3	1,712.5
Operating profit (loss)	17.9	(10.0)
Interest expense	42.3	43.4
Impairment of Magnetation investment		0.0
Other income (expense)	(4.1)	(2.6)
Income (loss) before income taxes	(28.5)	(56.0)
Income tax expense (benefit)	(6.1)	1.5
Equity in net income (loss) of subsidiaries	10.1	(247.0)
Net income (loss)	(12.3)	(304.5)
Less: Net income attributable to noncontrolling interests	0.0	0.0
Net income (loss) attributable to AK Steel Holding Corporation	(12.3)	(304.5)
Other comprehensive income (loss)	(1.0)	(10.2)
Comprehensive income (loss) attributable to AK Steel Holding Corporation	(13.3)	(314.7)
Current assets:
Cash and cash equivalents	27.0	28.5	85.9	27.0	35.0	28.5
Accounts receivable, net			452.9	411.9
Inventory, net			992.6	1,149.6
Other current assets			68.0	75.6
Total current assets			1,599.4	1,664.1
Property, plant and equipment			5,785.5	5,763.8
Accumulated depreciation			(4,264.1)	(4,218.0)
Property, plant and equipment, net			1,521.4	1,545.8
Investment in subsidiaries			1,380.2	1,346.0
Inter-company accounts			(3,390.4)	(3,600.9)
Other non-current assets			121.2	125.6
TOTAL ASSETS			1,231.8	1,080.6
Current liabilities:
Accounts payable			609.8	669.0
Accrued liabilities			252.4	242.3
Current portion of pension and other postretirement benefit obligations			78.1	77.3
Total current liabilities			940.3	988.6
Non-current liabilities:
Long-term debt			2,336.4	2,354.1
Pension and other postretirement benefit obligations			1,129.0	1,143.6
Other non-current liabilities			137.2	135.3
TOTAL LIABILITIES			4,542.9	4,621.6
Equity [Abstract]
Total stockholders' equity (deficit)			(3,311.1)	(3,541.0)
Noncontrolling interests			0.0	0.0
TOTAL EQUITY (DEFICIT)			(3,311.1)	(3,541.0)
TOTAL LIABILITIES AND EQUITY (DEFICIT)			1,231.8	1,080.6
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities	108.1	(40.7)
Cash flows from investing activities:
Capital investments	(27.2)	(26.2)
Other investing items, net	0.0	(5.7)
Net cash flows from investing activities	(27.2)	(31.9)
Cash flows from financing activities:
Net borrowings (payments) under credit facility	(30.0)	75.0
Inter-company activity	8.0	4.1
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	(22.0)	79.1
Net increase (decrease) in cash and cash equivalents	58.9	6.5
Cash and cash equivalents, beginning of period	27.0	28.5
Cash and cash equivalents, end of period	85.9	35.0
Guarantor Subsidiaries [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	58.7	65.8
Cost of products sold (exclusive of items shown separately below)	39.8	44.8
Selling and administrative expenses (exclusive of items shown separately below)	2.6	3.0
Depreciation	1.0	1.0
Pension and OPEB expense (income)	0.0	0.0
Total operating costs	43.4	48.8
Operating profit (loss)	15.3	17.0
Interest expense	0.0	0.0
Impairment of Magnetation investment		0.0
Other income (expense)	2.0	1.6
Income (loss) before income taxes	17.3	18.6
Income tax expense (benefit)	6.6	7.5
Equity in net income (loss) of subsidiaries	0.0	0.0
Net income (loss)	10.7	11.1
Less: Net income attributable to noncontrolling interests	0.0	0.0
Net income (loss) attributable to AK Steel Holding Corporation	10.7	11.1
Other comprehensive income (loss)	0.0	0.0
Comprehensive income (loss) attributable to AK Steel Holding Corporation	10.7	11.1
Current assets:
Cash and cash equivalents	0.0	0.5	0.1	0.0	0.1	0.5
Accounts receivable, net			25.8	23.5
Inventory, net			24.8	26.1
Other current assets			0.4	0.3
Total current assets			51.1	49.9
Property, plant and equipment			103.5	103.1
Accumulated depreciation			(76.5)	(75.4)
Property, plant and equipment, net			27.0	27.7
Investment in subsidiaries			0.0	0.0
Inter-company accounts			1,441.6	1,403.0
Other non-current assets			32.8	33.0
TOTAL ASSETS			1,552.5	1,513.6
Current liabilities:
Accounts payable			6.4	6.3
Accrued liabilities			2.2	2.8
Current portion of pension and other postretirement benefit obligations			0.0	0.0
Total current liabilities			8.6	9.1
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	9.1
Equity [Abstract]
Total stockholders' equity (deficit)			1,543.9	1,504.5
Noncontrolling interests			0.0	0.0
TOTAL EQUITY (DEFICIT)			1,543.9	1,504.5
TOTAL LIABILITIES AND EQUITY (DEFICIT)			1,552.5	1,513.6
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities	10.7	11.2
Cash flows from investing activities:
Capital investments	(0.7)	(0.8)
Other investing items, net	0.0	0.0
Net cash flows from investing activities	(0.7)	(0.8)
Cash flows from financing activities:
Net borrowings (payments) under credit facility	0.0	0.0
Inter-company activity	(9.9)	(10.8)
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	(9.9)	(10.8)
Net increase (decrease) in cash and cash equivalents	0.1	(0.4)
Cash and cash equivalents, beginning of period	0.0	0.5
Cash and cash equivalents, end of period	0.1	0.1
Other Non-Guarantor Subsidiaries [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	118.1	124.4
Cost of products sold (exclusive of items shown separately below)	87.7	95.1
Selling and administrative expenses (exclusive of items shown separately below)	6.5	6.6
Depreciation	6.3	6.0
Pension and OPEB expense (income)	0.0	0.0
Total operating costs	100.5	107.7
Operating profit (loss)	17.6	16.7
Interest expense	0.5	0.5
Impairment of Magnetation investment		(256.3)
Other income (expense)	1.4	(15.7)
Income (loss) before income taxes	18.5	(255.8)
Income tax expense (benefit)	0.2	(6.1)
Equity in net income (loss) of subsidiaries	0.0	0.0
Net income (loss)	18.3	(249.7)
Less: Net income attributable to noncontrolling interests	18.0	15.5
Net income (loss) attributable to AK Steel Holding Corporation	0.3	(265.2)
Other comprehensive income (loss)	1.5	(3.2)
Comprehensive income (loss) attributable to AK Steel Holding Corporation	1.8	(268.4)
Current assets:
Cash and cash equivalents	29.6	41.2	27.0	29.6	54.3	41.2
Accounts receivable, net			35.5	32.0
Inventory, net			63.2	60.6
Other current assets			3.4	2.5
Total current assets			129.1	124.7
Property, plant and equipment			599.4	599.1
Accumulated depreciation			(92.1)	(86.1)
Property, plant and equipment, net			507.3	513.0
Investment in subsidiaries			0.0	0.0
Inter-company accounts			(401.0)	(390.8)
Other non-current assets			35.2	33.1
TOTAL ASSETS			270.6	280.0
Current liabilities:
Accounts payable			27.8	29.1
Accrued liabilities			15.4	16.4
Current portion of pension and other postretirement benefit obligations			0.3	0.4
Total current liabilities			43.5	45.9
Non-current liabilities:
Long-term debt			0.0	0.0
Pension and other postretirement benefit obligations			3.4	3.3
Other non-current liabilities			1.2	1.1
TOTAL LIABILITIES			48.1	50.3
Equity [Abstract]
Total stockholders' equity (deficit)			(156.4)	(152.3)
Noncontrolling interests			378.9	382.0
TOTAL EQUITY (DEFICIT)			222.5	229.7
TOTAL LIABILITIES AND EQUITY (DEFICIT)			270.6	280.0
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities	15.1	32.3
Cash flows from investing activities:
Capital investments	(0.9)	(1.3)
Other investing items, net	(0.1)	0.1
Net cash flows from investing activities	(1.0)	(1.2)
Cash flows from financing activities:
Net borrowings (payments) under credit facility	0.0	0.0
Inter-company activity	4.4	0.3
SunCoke Middletown distributions to noncontrolling interest owners	(21.1)	(18.3)
Other financing items, net	0.0	0.0
Net cash flows from financing activities	(16.7)	(18.0)
Net increase (decrease) in cash and cash equivalents	(2.6)	13.1
Cash and cash equivalents, beginning of period	29.6	41.2
Cash and cash equivalents, end of period	27.0	54.3
Eliminations [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	(126.2)	(141.8)
Cost of products sold (exclusive of items shown separately below)	(113.3)	(140.7)
Selling and administrative expenses (exclusive of items shown separately below)	(11.4)	(12.9)
Depreciation	0.0	0.0
Pension and OPEB expense (income)	0.0	0.0
Total operating costs	(124.7)	(153.6)
Operating profit (loss)	(1.5)	11.8
Interest expense	0.0	0.0
Impairment of Magnetation investment		0.0
Other income (expense)	0.0	0.0
Income (loss) before income taxes	(1.5)	11.8
Income tax expense (benefit)	(0.6)	4.8
Equity in net income (loss) of subsidiaries	1.0	551.5
Net income (loss)	0.1	558.5
Less: Net income attributable to noncontrolling interests	0.0	0.0
Net income (loss) attributable to AK Steel Holding Corporation	0.1	558.5
Other comprehensive income (loss)	(0.5)	13.4
Comprehensive income (loss) attributable to AK Steel Holding Corporation	(0.4)	571.9
Current assets:
Cash and cash equivalents	0.0	0.0	0.0	0.0	$ 0.0	$ 0.0
Accounts receivable, net			(25.5)	(22.5)
Inventory, net			(11.7)	(10.0)
Other current assets			0.0	0.0
Total current assets			(37.2)	(32.5)
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			1,930.9	2,195.0
Inter-company accounts			29.2	25.3
Other non-current assets			0.0	0.0
TOTAL ASSETS			1,922.9	2,187.8
Current liabilities:
Accounts payable			(0.7)	(1.0)
Accrued liabilities			0.0	0.0
Current portion of pension and other postretirement benefit obligations			0.0	0.0
Total current liabilities			(0.7)	(1.0)
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			(0.7)	(1.0)
Equity [Abstract]
Total stockholders' equity (deficit)			1,923.6	2,188.8
Noncontrolling interests			0.0	0.0
TOTAL EQUITY (DEFICIT)			1,923.6	2,188.8
TOTAL LIABILITIES AND EQUITY (DEFICIT)			$ 1,922.9	$ 2,187.8
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities	4.1	(4.1)
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
Inter-company activity	(4.1)	4.1
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	(4.1)	4.1
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
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%