Supplemental Guarantor Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	$ 1,369.8	$ 1,508.7
Cost of products sold (exclusive of items shown separately below)	1,252.3	1,409.0
Selling and administrative expenses (exclusive of items shown separately below)	51.6	55.8
Depreciation	48.6	48.3
Pension and OPEB expense (income)	(15.9)	(8.5)
Total operating costs	1,336.6	1,504.6
Operating profit (loss)	33.2	4.1
Interest expense	31.0	16.2
Other income (expense)	1.8	0.9
Income (loss) before income taxes	4.0	(11.2)
Income tax provision (benefit)	(2.8)	(4.3)
Equity in net income (loss) of subsidiaries	0	0
Net income (loss)	6.8	(6.9)
Less: Net income attributable to noncontrolling interests	16.7	4.9
Net income (loss) attributable to AK Steel Holding Corporation	(9.9)	(11.8)
Other comprehensive income (loss)	(19.7)	(12.1)
Comprehensive income (loss) attributable to AK Steel Holding Corporation	(29.6)	(23.9)
Current assets:
Cash and cash equivalents	191.8	42.3
Accounts receivable, net	516.4		473.9
Inventory, net	664.0		609.2
Deferred tax assets, current	77.4		73.2
Other current assets	43.0		59.4
Total current assets	1,492.6		1,442.7
Property, plant and equipment	5,952.4		5,943.9
Accumulated depreciation	(3,980.1)		(3,931.6)
Property, plant and equipment, net	1,972.3		2,012.3
Other non-current assets:
Investment in AFSG Holdings, Inc.	55.6		55.6
Investment in Magnetation LLC	145.0		150.0
Investment in affiliates	0		0
Inter-company accounts	0		0
Goodwill	37.1		37.1
Deferred tax assets, non-current	88.4		88.2
Other non-current assets	115.1		117.2
TOTAL ASSETS	3,906.1		3,903.1
Current liabilities:
Accounts payable	592.4		538.3
Accrued liabilities	185.7		164.8
Current portion of long-term debt	0.7		0.7
Current portion of pension and other postretirement benefit obligations	109.8		108.6
Total current liabilities	888.6		812.4
Non-current liabilities:
Long-term debt	1,411.9		1,411.2
Pension and other postretirement benefit obligations	1,607.7		1,661.7
Other non-current liabilities	107.6		108.8
TOTAL LIABILITIES	4,015.8		3,994.1
Total stockholders' equity (deficit)	(531.6)		(505.3)
Noncontrolling interests	421.9		414.3
TOTAL EQUITY (DEFICIT)	(109.7)	360.6	(91.0)	377.2
TOTAL LIABILITIES AND EQUITY (DEFICIT)	3,906.1		3,903.1
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities	(7.0)	(150.0)
Cash flows from investing activities:
Capital investments	(16.5)	(29.7)
Other investing items, net	0.8	(0.2)
Net cash flows from investing activities	(15.7)	(29.9)
Cash flows from financing activities:
Net borrowings (repayments) under credit facility	0	(105.0)
Proceeds from issuance of long-term debt	0	373.3
Redemption of long-term debt	(0.2)	(73.5)
Debt issuance costs	(1.5)	(8.2)
Purchase of treasury stock	(0.6)	(1.7)
Common stock dividends paid	0	(5.5)
Inter-company activity	0	0
SunCoke Middletown distributions to noncontrolling interest owners	(9.1)
Other financing items, net	(1.1)	0.8
Net cash flows from financing activities	(12.5)	180.2
Net increase (decrease) in cash and cash equivalents	(35.2)	0.3
Cash and cash equivalents, beginning of period	227.0	42.0
Cash and cash equivalents, end of period	191.8	42.3
AK Holding [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	0	0
Cost of products sold (exclusive of items shown separately below)	0	0
Selling and administrative expenses (exclusive of items shown separately below)	1.5	1.4
Depreciation	0	0
Pension and OPEB expense (income)	0	0
Total operating costs	1.5	1.4
Operating profit (loss)	(1.5)	(1.4)
Interest expense	0	0
Other income (expense)	0	0
Income (loss) before income taxes	(1.5)	(1.4)
Income tax provision (benefit)	0	(0.6)
Equity in net income (loss) of subsidiaries	(8.4)	(11.0)
Net income (loss)	(9.9)	(11.8)
Less: Net income attributable to noncontrolling interests	0	0
Net income (loss) attributable to AK Steel Holding Corporation	(9.9)	(11.8)
Other comprehensive income (loss)	(19.7)	(12.1)
Comprehensive income (loss) attributable to AK Steel Holding Corporation	(29.6)	(23.9)
Current assets:
Cash and cash equivalents	0	0
Accounts receivable, net	0		0
Inventory, net	0		0
Deferred tax assets, current	0		0
Other current assets	0.2		0.2
Total current assets	0.2		0.2
Property, plant and equipment	0		0
Accumulated depreciation	0		0
Property, plant and equipment, net	0		0
Other non-current assets:
Investment in AFSG Holdings, Inc.	0		0
Investment in Magnetation LLC	0		0
Investment in affiliates	(2,739.2)		(2,660.7)
Inter-company accounts	2,207.4		2,155.2
Goodwill	0		0
Deferred tax assets, non-current	0		0
Other non-current assets	0		0
TOTAL ASSETS	(531.6)		(505.3)
Current liabilities:
Accounts payable	0		0
Accrued liabilities	0		0
Current portion of long-term debt	0		0
Current portion of pension and other postretirement benefit obligations	0		0
Total current liabilities	0		0
Non-current liabilities:
Long-term debt	0		0
Pension and other postretirement benefit obligations	0		0
Other non-current liabilities	0		0
TOTAL LIABILITIES	0		0
Total stockholders' equity (deficit)	(531.6)		(505.3)
Noncontrolling interests	0		0
TOTAL EQUITY (DEFICIT)	(531.6)		(505.3)
TOTAL LIABILITIES AND EQUITY (DEFICIT)	(531.6)		(505.3)
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities	(1.3)	(0.6)
Cash flows from investing activities:
Capital investments	0	0
Other investing items, net	0	0
Net cash flows from investing activities	0	0
Cash flows from financing activities:
Net borrowings (repayments) under credit facility		0
Proceeds from issuance of long-term debt		0
Redemption of long-term debt	0	0
Debt issuance costs	0	0
Purchase of treasury stock	(0.6)	(1.7)
Common stock dividends paid		(5.5)
Inter-company activity	1.9	7.8
SunCoke Middletown distributions to noncontrolling interest owners	0
Other financing items, net	0	0
Net cash flows from financing activities	1.3	0.6
Net increase (decrease) in cash and cash equivalents	0	0
Cash and cash equivalents, beginning of period	0	0
Cash and cash equivalents, end of period	0	0
AK Steel [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	1,283.7	1,433.2
Cost of products sold (exclusive of items shown separately below)	1,200.2	1,363.5
Selling and administrative expenses (exclusive of items shown separately below)	50.6	57.3
Depreciation	43.7	43.6
Pension and OPEB expense (income)	(15.9)	(8.5)
Total operating costs	1,278.6	1,455.9
Operating profit (loss)	5.1	(22.7)
Interest expense	30.7	16.0
Other income (expense)	(2.1)	(2.6)
Income (loss) before income taxes	(27.7)	(41.3)
Income tax provision (benefit)	(9.7)	(20.6)
Equity in net income (loss) of subsidiaries	9.6	9.7
Net income (loss)	(8.4)	(11.0)
Less: Net income attributable to noncontrolling interests	0	0
Net income (loss) attributable to AK Steel Holding Corporation	(8.4)	(11.0)
Other comprehensive income (loss)	(19.7)	(12.1)
Comprehensive income (loss) attributable to AK Steel Holding Corporation	(28.1)	(23.1)
Current assets:
Cash and cash equivalents	152.4	22.1
Accounts receivable, net	500.5		484.4
Inventory, net	588.9		504.2
Deferred tax assets, current	77.2		73.0
Other current assets	40.6		57.6
Total current assets	1,359.6		1,322.8
Property, plant and equipment	5,360.7		5,355.1
Accumulated depreciation	(3,885.6)		(3,841.9)
Property, plant and equipment, net	1,475.1		1,513.2
Other non-current assets:
Investment in AFSG Holdings, Inc.	0		0
Investment in Magnetation LLC	0		0
Investment in affiliates	1,352.4		1,337.4
Inter-company accounts	(3,136.6)		(3,066.5)
Goodwill	0		0
Deferred tax assets, non-current	88.0		87.9
Other non-current assets	76.9		79.8
TOTAL ASSETS	1,215.4		1,274.6
Current liabilities:
Accounts payable	548.2		494.8
Accrued liabilities	174.0		155.0
Current portion of long-term debt	0.7		0.7
Current portion of pension and other postretirement benefit obligations	109.3		108.1
Total current liabilities	832.2		758.6
Non-current liabilities:
Long-term debt	1,411.9		1,411.2
Pension and other postretirement benefit obligations	1,603.4		1,657.2
Other non-current liabilities	107.1		108.3
TOTAL LIABILITIES	3,954.6		3,935.3
Total stockholders' equity (deficit)	(2,739.2)		(2,660.7)
Noncontrolling interests	0		0
TOTAL EQUITY (DEFICIT)	(2,739.2)		(2,660.7)
TOTAL LIABILITIES AND EQUITY (DEFICIT)	1,215.4		1,274.6
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities	(29.7)	(174.7)
Cash flows from investing activities:
Capital investments	(13.6)	(9.7)
Other investing items, net	1.6	0
Net cash flows from investing activities	(12.0)	(9.7)
Cash flows from financing activities:
Net borrowings (repayments) under credit facility		(105.0)
Proceeds from issuance of long-term debt		373.3
Redemption of long-term debt	(0.2)	(73.5)
Debt issuance costs	(1.5)	(8.2)
Purchase of treasury stock	0	0
Common stock dividends paid		0
Inter-company activity	(6.0)	0
SunCoke Middletown distributions to noncontrolling interest owners	0
Other financing items, net	(1.8)	0
Net cash flows from financing activities	(9.5)	186.6
Net increase (decrease) in cash and cash equivalents	(51.2)	2.2
Cash and cash equivalents, beginning of period	203.6	19.9
Cash and cash equivalents, end of period	152.4	22.1
Other Non-Guarantor Subsidiaries [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	216.8	237.0
Cost of products sold (exclusive of items shown separately below)	173.8	196.4
Selling and administrative expenses (exclusive of items shown separately below)	9.7	8.3
Depreciation	4.9	4.7
Pension and OPEB expense (income)	0	0
Total operating costs	188.4	209.4
Operating profit (loss)	28.4	27.6
Interest expense	0.3	0.2
Other income (expense)	3.9	3.5
Income (loss) before income taxes	32.0	30.9
Income tax provision (benefit)	6.4	16.7
Equity in net income (loss) of subsidiaries	0	0
Net income (loss)	25.6	14.2
Less: Net income attributable to noncontrolling interests	16.7	4.9
Net income (loss) attributable to AK Steel Holding Corporation	8.9	9.3
Other comprehensive income (loss)	(1.1)	0.8
Comprehensive income (loss) attributable to AK Steel Holding Corporation	7.8	10.1
Current assets:
Cash and cash equivalents	39.4	20.2
Accounts receivable, net	68.8		54.3
Inventory, net	83.2		114.4
Deferred tax assets, current	0.2		0.2
Other current assets	2.2		1.6
Total current assets	193.8		193.9
Property, plant and equipment	591.7		588.8
Accumulated depreciation	(94.5)		(89.7)
Property, plant and equipment, net	497.2		499.1
Other non-current assets:
Investment in AFSG Holdings, Inc.	55.6		55.6
Investment in Magnetation LLC	145.0		150.0
Investment in affiliates	0		0
Inter-company accounts	873.9		843.8
Goodwill	37.1		37.1
Deferred tax assets, non-current	0.4		0.3
Other non-current assets	38.2		37.4
TOTAL ASSETS	1,841.2		1,817.2
Current liabilities:
Accounts payable	44.9		44.4
Accrued liabilities	11.7		9.8
Current portion of long-term debt	0		0
Current portion of pension and other postretirement benefit obligations	0.5		0.5
Total current liabilities	57.1		54.7
Non-current liabilities:
Long-term debt	0		0
Pension and other postretirement benefit obligations	4.3		4.5
Other non-current liabilities	0.5		0.5
TOTAL LIABILITIES	61.9		59.7
Total stockholders' equity (deficit)	1,357.4		1,343.2
Noncontrolling interests	421.9		414.3
TOTAL EQUITY (DEFICIT)	1,779.3		1,757.5
TOTAL LIABILITIES AND EQUITY (DEFICIT)	1,841.2		1,817.2
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities	36.3	23.3
Cash flows from investing activities:
Capital investments	(2.9)	(20.0)
Other investing items, net	(0.8)	(0.2)
Net cash flows from investing activities	(3.7)	(20.2)
Cash flows from financing activities:
Net borrowings (repayments) under credit facility		0
Proceeds from issuance of long-term debt		0
Redemption of long-term debt	0	0
Debt issuance costs	0	0
Purchase of treasury stock	0	0
Common stock dividends paid		0
Inter-company activity	(8.2)	(5.8)
SunCoke Middletown distributions to noncontrolling interest owners	(9.1)
Other financing items, net	0.7	0.8
Net cash flows from financing activities	(16.6)	(5.0)
Net increase (decrease) in cash and cash equivalents	16.0	(1.9)
Cash and cash equivalents, beginning of period	23.4	22.1
Cash and cash equivalents, end of period	39.4	20.2
Eliminations [Member]
Condensed Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net sales	(130.7)	(161.5)
Cost of products sold (exclusive of items shown separately below)	(121.7)	(150.9)
Selling and administrative expenses (exclusive of items shown separately below)	(10.2)	(11.2)
Depreciation	0	0
Pension and OPEB expense (income)	0	0
Total operating costs	(131.9)	(162.1)
Operating profit (loss)	1.2	0.6
Interest expense	0	0
Other income (expense)	0	0
Income (loss) before income taxes	1.2	0.6
Income tax provision (benefit)	0.5	0.2
Equity in net income (loss) of subsidiaries	(1.2)	1.3
Net income (loss)	(0.5)	1.7
Less: Net income attributable to noncontrolling interests	0	0
Net income (loss) attributable to AK Steel Holding Corporation	(0.5)	1.7
Other comprehensive income (loss)	20.8	11.3
Comprehensive income (loss) attributable to AK Steel Holding Corporation	20.3	13.0
Current assets:
Cash and cash equivalents	0	0
Accounts receivable, net	(52.9)		(64.8)
Inventory, net	(8.1)		(9.4)
Deferred tax assets, current	0		0
Other current assets	0		0
Total current assets	(61.0)		(74.2)
Property, plant and equipment	0		0
Accumulated depreciation	0		0
Property, plant and equipment, net	0		0
Other non-current assets:
Investment in AFSG Holdings, Inc.	0		0
Investment in Magnetation LLC	0		0
Investment in affiliates	1,386.8		1,323.3
Inter-company accounts	55.3		67.5
Goodwill	0		0
Deferred tax assets, non-current	0		0
Other non-current assets	0		0
TOTAL ASSETS	1,381.1		1,316.6
Current liabilities:
Accounts payable	(0.7)		(0.9)
Accrued liabilities	0		0
Current portion of long-term debt	0		0
Current portion of pension and other postretirement benefit obligations	0		0
Total current liabilities	(0.7)		(0.9)
Non-current liabilities:
Long-term debt	0		0
Pension and other postretirement benefit obligations	0		0
Other non-current liabilities	0		0
TOTAL LIABILITIES	(0.7)		(0.9)
Total stockholders' equity (deficit)	1,381.8		1,317.5
Noncontrolling interests	0		0
TOTAL EQUITY (DEFICIT)	1,381.8		1,317.5
TOTAL LIABILITIES AND EQUITY (DEFICIT)	1,381.1		1,316.6
Condensed Consolidated Statements of Cash Flows [Abstract]
Net cash flows from operating activities	(12.3)	2.0
Cash flows from investing activities:
Capital investments	0	0
Other investing items, net	0	0
Net cash flows from investing activities	0	0
Cash flows from financing activities:
Net borrowings (repayments) under credit facility		0
Proceeds from issuance of long-term debt		0
Redemption of long-term debt	0	0
Debt issuance costs	0	0
Purchase of treasury stock	0	0
Common stock dividends paid		0
Inter-company activity	12.3	(2.0)
SunCoke Middletown distributions to noncontrolling interest owners	0
Other financing items, net	0	0
Net cash flows from financing activities	12.3	(2.0)
Net increase (decrease) in cash and cash equivalents	0	0
Cash and cash equivalents, beginning of period	0	0
Cash and cash equivalents, end of period	$ 0	$ 0