Condensed Consolidating Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Condensed Consolidating Financial Statements
Ownership interest in subsidiaries (as a percent)	100.00%
Cash and equivalents	$ 477,861	$ 375,917	$ 357,809	$ 390,761
Investments in short-term commercial paper		31,520
Accounts receivable, net	753,301	642,363
Inventories	1,170,159	1,202,507
Other current assets	47,620	43,918
Total current assets	2,448,941	2,296,225
Property, plant and equipment, net	2,232,413	2,231,198
Intangible assets, net	408,832	416,635
Goodwill	736,912	738,542
Other assets, including investments in subs	129,476	132,816
Total assets	5,956,574	5,815,416	6,043,627
Accounts payable	410,632	360,097
Accrued expenses	172,473	219,949
Current maturities of long-term debt	231,582	29,631
Total current liabilities	814,687	609,677
Long-term debt	2,069,214	2,172,606
Other liabilities	566,669	556,477
Redeemable non controlling interest	99,414	98,814
Common stock	639	637
Treasury stock	(719,076)	(720,479)
Additional paid-in capital	1,045,973	1,037,687
Retained earnings (deficit)	2,111,597	2,087,620
Total Steel Dynamics, Inc. equity	2,439,133	2,405,465
Noncontrolling interests	(32,543)	(27,623)
Total equity	2,406,590	2,377,842
Total liabilities and equity	5,956,574	5,815,416
Parent
Condensed Consolidating Financial Statements
Cash and equivalents	441,989	322,707	273,952	301,073
Investments in short-term commercial paper		31,520
Accounts receivable, net	290,332	277,428
Inventories	604,370	564,882
Other current assets	51,597	51,268
Total current assets	1,388,288	1,247,805
Property, plant and equipment, net	1,018,938	1,017,587
Other assets, including investments in subs	2,745,488	2,768,360
Total assets	5,152,714	5,033,752
Accounts payable	186,419	150,191
Accrued expenses	104,882	144,719
Current maturities of long-term debt	215,991	14,237
Total current liabilities	507,292	309,147
Long-term debt	2,033,955	2,140,958
Other liabilities	172,334	178,182
Common stock	639	637
Treasury stock	(719,076)	(720,479)
Additional paid-in capital	1,045,973	1,037,687
Retained earnings (deficit)	2,111,597	2,087,620
Total Steel Dynamics, Inc. equity	2,439,133	2,405,465
Total equity	2,439,133	2,405,465
Total liabilities and equity	5,152,714	5,033,752
Guarantors
Condensed Consolidating Financial Statements
Cash and equivalents	22,546	41,675	67,989	58,699
Accounts receivable, net	874,233	772,868
Inventories	482,512	536,331
Other current assets	8,443	7,253
Total current assets	1,387,734	1,358,127
Property, plant and equipment, net	657,918	664,332
Intangible assets, net	408,832	416,635
Goodwill	736,912	738,542
Other assets, including investments in subs	25,784	30,862
Total assets	3,217,180	3,208,498
Accounts payable	232,442	219,415
Accrued expenses	91,586	98,484
Current maturities of long-term debt	300	300
Total current liabilities	324,328	318,199
Other liabilities	2,067,830	2,087,957
Common stock	33,896	33,896
Additional paid-in capital	117,737	117,737
Retained earnings (deficit)	673,389	650,709
Total Steel Dynamics, Inc. equity	825,022	802,342
Total equity	825,022	802,342
Total liabilities and equity	3,217,180	3,208,498
Combined Non-Guarantors
Condensed Consolidating Financial Statements
Cash and equivalents	13,326	11,535	15,868	30,989
Accounts receivable, net	25,813	11,293
Inventories	88,668	107,422
Other current assets	6,298	4,006
Total current assets	134,105	134,256
Property, plant and equipment, net	558,130	551,903
Other assets, including investments in subs	8,724	9,189
Total assets	700,959	695,348
Accounts payable	63,767	56,472
Accrued expenses	10,179	9,877
Current maturities of long-term debt	41,517	52,595
Total current liabilities	115,463	118,944
Long-term debt	198,695	169,223
Other liabilities	49,376	41,581
Redeemable non controlling interest	99,414	98,814
Common stock	18,121	18,121
Additional paid-in capital	479,648	476,677
Retained earnings (deficit)	(227,215)	(200,389)
Total Steel Dynamics, Inc. equity	270,554	294,409
Noncontrolling interests	(32,543)	(27,623)
Total equity	238,011	266,786
Total liabilities and equity	700,959	695,348
Consolidating Adjustments
Condensed Consolidating Financial Statements
Accounts receivable, net	(437,077)	(419,226)
Inventories	(5,391)	(6,128)
Other current assets	(18,718)	(18,609)
Total current assets	(461,186)	(443,963)
Property, plant and equipment, net	(2,573)	(2,624)
Other assets, including investments in subs	(2,650,520)	(2,675,595)
Total assets	(3,114,279)	(3,122,182)
Accounts payable	(71,996)	(65,981)
Accrued expenses	(34,174)	(33,131)
Current maturities of long-term debt	(26,226)	(37,501)
Total current liabilities	(132,396)	(136,613)
Long-term debt	(163,436)	(137,575)
Other liabilities	(1,722,871)	(1,751,243)
Common stock	(52,017)	(52,017)
Additional paid-in capital	(597,385)	(594,414)
Retained earnings (deficit)	(446,174)	(450,320)
Total Steel Dynamics, Inc. equity	(1,095,576)	(1,096,751)
Total equity	(1,095,576)	(1,096,751)
Total liabilities and equity	$ (3,114,279)	$ (3,122,182)