Condensed Consolidating Information (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Condensed Consolidating Financial Statements
Ownership interest in subsidiaries (as a percent)	100.00%
Cash and equivalents	$ 375,917	$ 390,761	$ 186,513	$ 9,008
Investments in short-term commercial paper	31,520	84,830
Accounts receivable, net	642,363	722,791
Inventories	1,202,507	1,199,584
Other current assets	43,918	57,292
Total current assets	2,296,225	2,455,258
Property, plant and equipment, net	2,231,198	2,193,745
Intangible assets, net	416,635	450,893
Goodwill	738,542	745,066
Other assets, including investments in subs	132,816	134,264
Total assets	5,815,416	5,979,226	5,589,934
Accounts payable	360,097	420,824
Accrued expenses	219,949	235,515
Current maturities of long-term debt	29,631	444,078
Total current liabilities	609,677	1,100,417
Long-term debt	2,172,606	1,936,022
Other liabilities	556,477	572,193
Redeemable non controlling interest	98,814	70,694
Common stock	637	636
Treasury stock	(720,479)	(722,653)
Additional paid-in capital	1,037,687	1,026,157
Retained earnings (deficit)	2,087,620	2,011,801
Total Steel Dynamics, Inc. equity	2,405,465	2,315,941
Noncontrolling interests	(27,623)	(16,041)
Total equity	2,377,842	2,299,900	2,076,835	2,003,265
Total liabilities and equity	5,815,416	5,979,226
Parent
Condensed Consolidating Financial Statements
Cash and equivalents	322,707	301,073	173,563	430
Investments in short-term commercial paper	31,520	84,830
Accounts receivable, net	277,428	319,995
Inventories	564,882	584,079
Other current assets	51,268	83,116
Total current assets	1,247,805	1,373,093
Property, plant and equipment, net	1,017,587	1,059,011
Other assets, including investments in subs	2,768,360	2,791,718
Total assets	5,033,752	5,223,822
Accounts payable	150,191	155,220
Accrued expenses	144,719	157,868
Current maturities of long-term debt	14,237	439,199
Total current liabilities	309,147	752,287
Long-term debt	2,140,958	1,905,199
Other liabilities	178,182	250,395
Common stock	637	636
Treasury stock	(720,479)	(722,653)
Additional paid-in capital	1,037,687	1,026,157
Retained earnings (deficit)	2,087,620	2,011,801
Total Steel Dynamics, Inc. equity	2,405,465	2,315,941
Total equity	2,405,465	2,315,941
Total liabilities and equity	5,033,752	5,223,822
Guarantors
Condensed Consolidating Financial Statements
Cash and equivalents	41,675	58,699	10,628	6,362
Accounts receivable, net	772,868	726,192
Inventories	536,331	533,022
Other current assets	7,253	4,119
Total current assets	1,358,127	1,322,032
Property, plant and equipment, net	664,332	660,048
Intangible assets, net	416,635	450,893
Goodwill	738,542	745,066
Other assets, including investments in subs	30,862	33,507
Total assets	3,208,498	3,211,546
Accounts payable	219,415	273,049
Accrued expenses	98,484	98,498
Current maturities of long-term debt	300	300
Total current liabilities	318,199	371,847
Other liabilities	2,087,957	2,132,778
Common stock	33,896	33,896
Additional paid-in capital	117,737	117,737
Retained earnings (deficit)	650,709	555,288
Total Steel Dynamics, Inc. equity	802,342	706,921
Total equity	802,342	706,921
Total liabilities and equity	3,208,498	3,211,546
Combined Non-Guarantors
Condensed Consolidating Financial Statements
Cash and equivalents	11,535	30,989	2,322	2,216
Accounts receivable, net	11,293	8,971
Inventories	107,422	85,046
Other current assets	4,006	2,304
Total current assets	134,256	127,310
Property, plant and equipment, net	551,903	477,514
Other assets, including investments in subs	9,189	9,853
Total assets	695,348	614,677
Accounts payable	56,472	23,195
Accrued expenses	9,877	10,012
Current maturities of long-term debt	52,595	53,551
Total current liabilities	118,944	86,758
Long-term debt	169,223	169,797
Other liabilities	41,581	51,625
Redeemable non controlling interest	98,814	70,694
Common stock	18,121	18,121
Additional paid-in capital	476,677	347,151
Retained earnings (deficit)	(200,389)	(113,428)
Total Steel Dynamics, Inc. equity	294,409	251,844
Noncontrolling interests	(27,623)	(16,041)
Total equity	266,786	235,803
Total liabilities and equity	695,348	614,677
Consolidating Adjustments
Condensed Consolidating Financial Statements
Accounts receivable, net	(419,226)	(332,367)
Inventories	(6,128)	(2,563)
Other current assets	(18,609)	(32,247)
Total current assets	(443,963)	(367,177)
Property, plant and equipment, net	(2,624)	(2,828)
Other assets, including investments in subs	(2,675,595)	(2,700,814)
Total assets	(3,122,182)	(3,070,819)
Accounts payable	(65,981)	(30,640)
Accrued expenses	(33,131)	(30,863)
Current maturities of long-term debt	(37,501)	(48,972)
Total current liabilities	(136,613)	(110,475)
Long-term debt	(137,575)	(138,974)
Other liabilities	(1,751,243)	(1,862,605)
Common stock	(52,017)	(52,017)
Additional paid-in capital	(594,414)	(464,888)
Retained earnings (deficit)	(450,320)	(441,860)
Total Steel Dynamics, Inc. equity	(1,096,751)	(958,765)
Total equity	(1,096,751)	(958,765)
Total liabilities and equity	$ (3,122,182)	$ (3,070,819)