Condensed Consolidating Information (Details 2) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Condensed Consolidating Statements of Operations
Net sales	$ 1,950,923	$ 2,339,016	$ 6,003,365	$ 6,238,859
Costs of goods sold	1,722,197	2,050,504	5,415,854	5,564,272
Gross profit	228,726	288,512	587,511	674,587
Selling, general and administrative	97,697	99,869	279,152	273,107
Operating income	131,029	188,643	308,359	401,480
Interest expense, net of capitalized interest	37,084	31,904	117,334	92,523
Other (income) expense, net	239	22,072	15,219	19,687
Income before income taxes	93,706	134,667	175,806	289,270
Income taxes (benefit)	34,839	47,010	64,660	101,574
Net income	58,867	87,657	111,146	187,696
Net loss attributable to noncontrolling interests	1,750	3,516	11,782	14,359
Net income attributable to Steel Dynamics, Inc.	60,617	91,173	122,928	202,055
Reportable legal entity | Parent
Condensed Consolidating Statements of Operations
Net sales	735,250	1,044,207	2,300,024	2,915,339
Costs of goods sold	619,833	856,372	1,982,667	2,477,791
Gross profit	115,417	187,835	317,357	437,548
Selling, general and administrative	35,235	40,932	95,883	105,185
Operating income	80,182	146,903	221,474	332,363
Interest expense, net of capitalized interest	18,312	18,965	57,015	57,357
Other (income) expense, net	252	22,548	15,131	20,016
Income before income taxes	61,618	105,390	149,328	254,990
Income taxes (benefit)	20,169	30,605	35,207	64,480
Net income	41,449	74,785	114,121	190,510
Equity in net loss of subsidiaries	19,168	16,388	8,807	11,545
Net income attributable to Steel Dynamics, Inc.	60,617	91,173	122,928	202,055
Reportable legal entity | Guarantors
Condensed Consolidating Statements of Operations
Net sales	2,103,824	2,609,731	6,447,974	7,035,579
Costs of goods sold	1,965,364	2,469,396	6,056,514	6,668,153
Gross profit	138,460	140,335	391,460	367,426
Selling, general and administrative	64,259	59,878	188,028	170,578
Operating income	74,201	80,457	203,432	196,848
Interest expense, net of capitalized interest	18,138	12,238	58,354	33,093
Other (income) expense, net	(631)	(202)	(597)	147
Income before income taxes	56,694	68,421	145,675	163,608
Income taxes (benefit)	21,697	24,754	53,903	58,772
Net income	34,997	43,667	91,772	104,836
Net income attributable to Steel Dynamics, Inc.	34,997	43,667	91,772	104,836
Reportable legal entity | Combined Non-Guarantors
Condensed Consolidating Statements of Operations
Net sales	96,204	150,551	302,657	377,639
Costs of goods sold	99,864	162,094	352,980	427,029
Gross profit	(3,660)	(11,543)	(50,323)	(49,390)
Selling, general and administrative	2,845	3,601	8,878	10,415
Operating income	(6,505)	(15,144)	(59,201)	(59,805)
Interest expense, net of capitalized interest	1,692	1,958	5,084	5,824
Other (income) expense, net	(440)	(1,530)	(2,434)	(4,226)
Income before income taxes	(7,757)	(15,572)	(61,851)	(61,403)
Income taxes (benefit)	(421)	(561)	(4,047)	949
Net income	(7,336)	(15,011)	(57,804)	(62,352)
Net loss attributable to noncontrolling interests	1,750	3,516	11,782	14,359
Net income attributable to Steel Dynamics, Inc.	(5,586)	(11,495)	(46,022)	(47,993)
Consolidating Adjustments
Condensed Consolidating Statements of Operations
Net sales	(984,355)	(1,465,473)	(3,047,290)	(4,089,698)
Costs of goods sold	(962,864)	(1,437,358)	(2,976,307)	(4,008,701)
Gross profit	(21,491)	(28,115)	(70,983)	(80,997)
Selling, general and administrative	(4,642)	(4,542)	(13,637)	(13,071)
Operating income	(16,849)	(23,573)	(57,346)	(67,926)
Interest expense, net of capitalized interest	(1,058)	(1,257)	(3,119)	(3,751)
Other (income) expense, net	1,058	1,256	3,119	3,750
Income before income taxes	(16,849)	(23,572)	(57,346)	(67,925)
Income taxes (benefit)	(6,606)	(7,788)	(20,403)	(22,627)
Net income	(10,243)	(15,784)	(36,943)	(45,298)
Equity in net loss of subsidiaries	(19,168)	(16,388)	(8,807)	(11,545)
Net income attributable to Steel Dynamics, Inc.	$ (29,411)	$ (32,172)	$ (45,750)	$ (56,843)