Condensed Consolidating Information (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011
Condensed Consolidating Financial Statements
Ownership interest in subsidiaries (as a percent)	100.00%
Cash and equivalents	$ 370,056	$ 243,753	$ 375,917	$ 287,120	$ 420,938	$ 390,761
Investments in short-term commercial paper			31,520
Accounts receivable, net	772,873		642,363
Inventories	1,192,032		1,202,507
Other current assets	50,354		43,918
Total current assets	2,385,315		2,296,225
Property, plant and equipment, net	2,241,067		2,231,198
Intangible assets, net	393,531		416,635
Goodwill	733,650		738,542
Other assets, including investments in subs	120,742		132,816
Total assets	5,874,305		5,815,416	5,775,516
Accounts payable	426,406		360,097
Accrued expenses	192,455		219,949
Current maturities of long-term debt	335,341		29,631
Total current liabilities	954,202		609,677
Long-term debt	1,773,619		2,172,606
Other liabilities	588,696		556,477
Redeemable noncontrolling interests	110,054		98,814
Common stock	641		637
Treasury stock	(718,373)		(720,479)
Additional paid-in capital	1,060,780		1,037,687
Retained earnings (deficit)	2,149,389		2,087,620
Total Steel Dynamics, Inc. equity	2,492,437		2,405,465
Noncontrolling interests	(44,703)		(27,623)
Total equity	2,447,734		2,377,842
Total liabilities and equity	5,874,305		5,815,416
Parent
Condensed Consolidating Financial Statements
Cash and equivalents	322,212		322,707	242,963		301,073
Investments in short-term commercial paper			31,520
Accounts receivable, net	311,333		277,428
Inventories	622,008		564,882
Other current assets	57,805		51,268
Total current assets	1,313,358		1,247,805
Property, plant and equipment, net	1,042,112		1,017,587
Other assets, including investments in subs	2,621,656		2,768,360
Total assets	4,977,126		5,033,752
Accounts payable	155,868		150,191
Accrued expenses	119,919		144,719
Current maturities of long-term debt	312,076		14,237
Total current liabilities	587,863		309,147
Long-term debt	1,732,445		2,140,958
Other liabilities	164,381		178,182
Common stock	641		637
Treasury stock	(718,373)		(720,479)
Additional paid-in capital	1,060,780		1,037,687
Retained earnings (deficit)	2,149,389		2,087,620
Total Steel Dynamics, Inc. equity	2,492,437		2,405,465
Total equity	2,492,437		2,405,465
Total liabilities and equity	4,977,126		5,033,752
Guarantors
Condensed Consolidating Financial Statements
Cash and equivalents	37,710		41,675	26,990		58,699
Accounts receivable, net	900,928		772,868
Inventories	471,537		536,331
Other current assets	7,301		7,253
Total current assets	1,417,476		1,358,127
Property, plant and equipment, net	634,552		664,332
Intangible assets, net	393,531		416,635
Goodwill	733,650		738,542
Other assets, including investments in subs	25,367		30,862
Total assets	3,204,576		3,208,498
Accounts payable	257,009		219,415
Accrued expenses	105,444		98,484
Current maturities of long-term debt	300		300
Total current liabilities	362,753		318,199
Other liabilities	1,968,990		2,087,957
Common stock	33,896		33,896
Additional paid-in capital	117,737		117,737
Retained earnings (deficit)	721,200		650,709
Total Steel Dynamics, Inc. equity	872,833		802,342
Total equity	872,833		802,342
Total liabilities and equity	3,204,576		3,208,498
Combined Non-Guarantors
Condensed Consolidating Financial Statements
Cash and equivalents	10,134		11,535	17,167		30,989
Accounts receivable, net	35,210		11,293
Inventories	102,803		107,422
Other current assets	3,987		4,006
Total current assets	152,134		134,256
Property, plant and equipment, net	566,873		551,903
Other assets, including investments in subs	8,266		9,189
Total assets	727,273		695,348
Accounts payable	83,830		56,472
Accrued expenses	10,014		9,877
Current maturities of long-term debt	46,060		52,595
Total current liabilities	139,904		118,944
Long-term debt	202,825		169,223
Other liabilities	44,567		41,581
Redeemable noncontrolling interests	110,054		98,814
Common stock	18,121		18,121
Additional paid-in capital	525,230		476,677
Retained earnings (deficit)	(268,725)		(200,389)
Total Steel Dynamics, Inc. equity	274,626		294,409
Noncontrolling interests	(44,703)		(27,623)
Total equity	229,923		266,786
Total liabilities and equity	727,273		695,348
Consolidating Adjustments
Condensed Consolidating Financial Statements
Accounts receivable, net	(474,598)		(419,226)
Inventories	(4,316)		(6,128)
Other current assets	(18,739)		(18,609)
Total current assets	(497,653)		(443,963)
Property, plant and equipment, net	(2,470)		(2,624)
Other assets, including investments in subs	(2,534,547)		(2,675,595)
Total assets	(3,034,670)		(3,122,182)
Accounts payable	(70,301)		(65,981)
Accrued expenses	(42,922)		(33,131)
Current maturities of long-term debt	(23,095)		(37,501)
Total current liabilities	(136,318)		(136,613)
Long-term debt	(161,651)		(137,575)
Other liabilities	(1,589,242)		(1,751,243)
Common stock	(52,017)		(52,017)
Additional paid-in capital	(642,967)		(594,414)
Retained earnings (deficit)	(452,475)		(450,320)
Total Steel Dynamics, Inc. equity	(1,147,459)		(1,096,751)
Total equity	(1,147,459)		(1,096,751)
Total liabilities and equity	$ (3,034,670)		$ (3,122,182)