Condensed Consolidating Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2010
Condensed Consolidating Financial Statements
Ownership interest in subsidiaries (as a percent)	100.00%
Cash and equivalents	$ 357,809	$ 390,761	$ 160,810	$ 186,513
Investments in short-term commercial paper	64,982	84,830
Accounts receivable, net	789,174	722,791
Inventories	1,270,941	1,199,584
Other current assets	43,951	57,292
Total current assets	2,526,857	2,455,258
Property, plant and equipment, net	2,199,509	2,193,745
Intangible assets, net	442,277	450,893
Goodwill	743,441	745,066
Other assets, including investments in subs	131,543	134,264
Total assets	6,043,627	5,979,226	5,827,605
Accounts payable	472,257	420,824
Accrued expenses	209,978	235,515
Current maturities of long-term debt	439,633	444,078
Total current liabilities	1,121,868	1,100,417
Long-term debt	1,932,019	1,936,022
Other liabilities	583,255	572,193
Redeemable noncontrolling interest	73,924	70,694
Common stock	636	636
Treasury stock	(721,024)	(722,653)
Additional paid-in capital	1,031,043	1,026,157
Retained earnings (deficit)	2,035,569	2,011,801
Total Steel Dynamics, Inc. equity	2,346,224	2,315,941
Noncontrolling interests	(13,663)	(16,041)
Total equity	2,332,561	2,299,900
Total liabilities and equity	6,043,627	5,979,226
Parent
Condensed Consolidating Financial Statements
Cash and equivalents	273,952	301,073	147,752	173,563
Investments in short-term commercial paper	64,982	84,830
Accounts receivable, net	314,044	319,995
Inventories	695,432	584,079
Other current assets	53,772	83,116
Total current assets	1,402,182	1,373,093
Property, plant and equipment, net	1,043,186	1,059,011
Other assets, including investments in subs	2,788,045	2,791,718
Total assets	5,233,413	5,223,822
Accounts payable	158,774	155,220
Accrued expenses	139,704	157,868
Current maturities of long-term debt	434,489	439,199
Total current liabilities	732,967	752,287
Long-term debt	1,901,644	1,905,199
Other liabilities	252,578	250,395
Common stock	636	636
Treasury stock	(721,024)	(722,653)
Additional paid-in capital	1,031,043	1,026,157
Retained earnings (deficit)	2,035,569	2,011,801
Total Steel Dynamics, Inc. equity	2,346,224	2,315,941
Total equity	2,346,224	2,315,941
Total liabilities and equity	5,233,413	5,223,822
Guarantors
Condensed Consolidating Financial Statements
Cash and equivalents	67,989	58,699	8,976	10,628
Accounts receivable, net	801,967	726,192
Inventories	508,592	533,022
Other current assets	6,271	4,119
Total current assets	1,384,819	1,322,032
Property, plant and equipment, net	663,035	660,048
Intangible assets, net	442,277	450,893
Goodwill	743,441	745,066
Other assets, including investments in subs	32,191	33,507
Total assets	3,265,763	3,211,546
Accounts payable	314,741	273,049
Accrued expenses	93,131	98,498
Current maturities of long-term debt	300	300
Total current liabilities	408,172	371,847
Other liabilities	2,124,643	2,132,778
Common stock	33,896	33,896
Additional paid-in capital	117,737	117,737
Retained earnings (deficit)	581,315	555,288
Total Steel Dynamics, Inc. equity	732,948	706,921
Total equity	732,948	706,921
Total liabilities and equity	3,265,763	3,211,546
Combined Non-Guarantors
Condensed Consolidating Financial Statements
Cash and equivalents	15,868	30,989	4,082	2,322
Accounts receivable, net	11,469	8,971
Inventories	71,260	85,046
Other current assets	1,944	2,304
Total current assets	100,541	127,310
Property, plant and equipment, net	496,065	477,514
Other assets, including investments in subs	9,555	9,853
Total assets	606,161	614,677
Accounts payable	22,117	23,195
Accrued expenses	9,156	10,012
Current maturities of long-term debt	42,072	53,551
Total current liabilities	73,345	86,758
Long-term debt	178,345	169,797
Other liabilities	38,110	51,625
Redeemable noncontrolling interest	73,924	70,694
Common stock	18,121	18,121
Additional paid-in capital	368,598	347,151
Retained earnings (deficit)	(130,619)	(113,428)
Total Steel Dynamics, Inc. equity	256,100	251,844
Noncontrolling interests	(13,663)	(16,041)
Total equity	242,437	235,803
Total liabilities and equity	606,161	614,677
Consolidating Adjustments
Condensed Consolidating Financial Statements
Accounts receivable, net	(338,306)	(332,367)
Inventories	(4,343)	(2,563)
Other current assets	(18,036)	(32,247)
Total current assets	(360,685)	(367,177)
Property, plant and equipment, net	(2,777)	(2,828)
Other assets, including investments in subs	(2,698,248)	(2,700,814)
Total assets	(3,061,710)	(3,070,819)
Accounts payable	(23,375)	(30,640)
Accrued expenses	(32,013)	(30,863)
Current maturities of long-term debt	(37,228)	(48,972)
Total current liabilities	(92,616)	(110,475)
Long-term debt	(147,970)	(138,974)
Other liabilities	(1,832,076)	(1,862,605)
Common stock	(52,017)	(52,017)
Additional paid-in capital	(486,335)	(464,888)
Retained earnings (deficit)	(450,696)	(441,860)
Total Steel Dynamics, Inc. equity	(989,048)	(958,765)
Total equity	(989,048)	(958,765)
Total liabilities and equity	$ (3,061,710)	$ (3,070,819)