Condensed Consolidating Information (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011
Condensed Consolidating Financial Statements
Ownership interest in subsidiaries (as a percent)	100.00%
Cash and equivalents	$ 243,753	$ 477,861	$ 375,917	$ 420,938	$ 357,809	$ 390,761
Investments in short-term commercial paper			31,520
Accounts receivable, net	773,127		642,363
Inventories	1,168,499		1,202,507
Other current assets	56,710		43,918
Total current assets	2,242,089		2,296,225
Property, plant and equipment, net	2,232,852		2,231,198
Intangible assets, net	401,104		416,635
Goodwill	735,281		738,542
Other assets, including investments in subs	124,584		132,816
Total assets	5,735,910		5,815,416	5,971,380
Accounts payable	364,330		360,097
Accrued expenses	181,278		219,949
Current maturities of long-term debt	324,241		29,631
Total current liabilities	869,849		609,677
Long-term debt	1,774,243		2,172,606
Other liabilities	576,347		556,477
Redeemable noncontrolling interests	104,734		98,814
Common stock	640		637
Treasury stock	(718,373)		(720,479)
Additional paid-in capital	1,050,470		1,037,687
Retained earnings (deficit)	2,116,262		2,087,620
Total Steel Dynamics, Inc. equity	2,448,999		2,405,465
Noncontrolling interests	(38,262)		(27,623)
Total equity	2,410,737		2,377,842
Total liabilities and equity	5,735,910		5,815,416
Parent
Condensed Consolidating Financial Statements
Cash and equivalents	213,717		322,707	304,807		301,073
Investments in short-term commercial paper			31,520
Accounts receivable, net	328,445		277,428
Inventories	610,160		564,882
Other current assets	63,513		51,268
Total current assets	1,215,835		1,247,805
Property, plant and equipment, net	1,029,669		1,017,587
Other assets, including investments in subs	2,672,022		2,768,360
Total assets	4,917,526		5,033,752
Accounts payable	143,874		150,191
Accrued expenses	111,890		144,719
Current maturities of long-term debt	308,628		14,237
Total current liabilities	564,392		309,147
Long-term debt	1,739,453		2,140,958
Other liabilities	164,682		178,182
Common stock	640		637
Treasury stock	(718,373)		(720,479)
Additional paid-in capital	1,050,470		1,037,687
Retained earnings (deficit)	2,116,262		2,087,620
Total Steel Dynamics, Inc. equity	2,448,999		2,405,465
Total equity	2,448,999		2,405,465
Total liabilities and equity	4,917,526		5,033,752
Guarantors
Condensed Consolidating Financial Statements
Cash and equivalents	14,726		41,675	105,248		58,699
Accounts receivable, net	847,046		772,868
Inventories	470,759		536,331
Other current assets	6,012		7,253
Total current assets	1,338,543		1,358,127
Property, plant and equipment, net	649,739		664,332
Intangible assets, net	401,104		416,635
Goodwill	735,281		738,542
Other assets, including investments in subs	25,627		30,862
Total assets	3,150,294		3,208,498
Accounts payable	208,660		219,415
Accrued expenses	96,622		98,484
Current maturities of long-term debt	300		300
Total current liabilities	305,582		318,199
Other liabilities	1,999,617		2,087,957
Common stock	33,896		33,896
Additional paid-in capital	117,737		117,737
Retained earnings (deficit)	693,462		650,709
Total Steel Dynamics, Inc. equity	845,095		802,342
Total equity	845,095		802,342
Total liabilities and equity	3,150,294		3,208,498
Combined Non-Guarantors
Condensed Consolidating Financial Statements
Cash and equivalents	15,310		11,535	10,883		30,989
Accounts receivable, net	43,406		11,293
Inventories	90,197		107,422
Other current assets	5,860		4,006
Total current assets	154,773		134,256
Property, plant and equipment, net	555,965		551,903
Other assets, including investments in subs	8,516		9,189
Total assets	719,254		695,348
Accounts payable	79,910		56,472
Accrued expenses	8,741		9,877
Current maturities of long-term debt	41,163		52,595
Total current liabilities	129,814		118,944
Long-term debt	196,806		169,223
Other liabilities	51,957		41,581
Redeemable noncontrolling interests	104,734		98,814
Common stock	18,121		18,121
Additional paid-in capital	502,450		476,677
Retained earnings (deficit)	(246,366)		(200,389)
Total Steel Dynamics, Inc. equity	274,205		294,409
Noncontrolling interests	(38,262)		(27,623)
Total equity	235,943		266,786
Total liabilities and equity	719,254		695,348
Consolidating Adjustments
Condensed Consolidating Financial Statements
Accounts receivable, net	(445,770)		(419,226)
Inventories	(2,617)		(6,128)
Other current assets	(18,675)		(18,609)
Total current assets	(467,062)		(443,963)
Property, plant and equipment, net	(2,521)		(2,624)
Other assets, including investments in subs	(2,581,581)		(2,675,595)
Total assets	(3,051,164)		(3,122,182)
Accounts payable	(68,114)		(65,981)
Accrued expenses	(35,975)		(33,131)
Current maturities of long-term debt	(25,850)		(37,501)
Total current liabilities	(129,939)		(136,613)
Long-term debt	(162,016)		(137,575)
Other liabilities	(1,639,909)		(1,751,243)
Common stock	(52,017)		(52,017)
Additional paid-in capital	(620,187)		(594,414)
Retained earnings (deficit)	(447,096)		(450,320)
Total Steel Dynamics, Inc. equity	(1,119,300)		(1,096,751)
Total equity	(1,119,300)		(1,096,751)
Total liabilities and equity	$ (3,051,164)		$ (3,122,182)