Condensed Consolidating Information (Details 2) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Condensed Consolidating Statements of Operations
Net sales	$ 2,005,007	$ 2,069,761	$ 4,052,442	$ 3,899,843
Costs of goods sold	1,833,264	1,846,990	3,693,657	3,513,768
Gross profit	171,743	222,771	358,785	386,075
Selling, general and administrative	94,184	90,866	181,455	173,238
Operating income	77,559	131,905	177,330	212,837
Interest expense, net of capitalized interest	37,163	30,050	80,250	60,619
Other (income) expense, net	(1,212)	(1,754)	14,980	(2,385)
Income before income taxes	41,608	103,609	82,100	154,603
Income taxes (benefit)	16,283	37,268	29,821	54,564
Net income	25,325	66,341	52,279	100,039
Net loss attributable to noncontrolling interests	6,225	5,962	10,032	10,843
Net income attributable to Steel Dynamics, Inc.	31,550	72,303	62,311	110,882
Reportable legal entity | Parent
Condensed Consolidating Statements of Operations
Net sales	766,056	1,007,569	1,564,774	1,871,132
Costs of goods sold	669,259	860,024	1,362,834	1,621,419
Gross profit	96,797	147,545	201,940	249,713
Selling, general and administrative	29,905	35,419	60,648	64,253
Operating income	66,892	112,126	141,292	185,460
Interest expense, net of capitalized interest	18,166	19,031	38,703	38,392
Other (income) expense, net	(773)	(1,556)	14,879	(2,532)
Income before income taxes	49,499	94,651	87,710	149,600
Income taxes (benefit)	8,097	23,950	15,038	33,875
Net income	41,402	70,701	72,672	115,725
Equity in net loss of subsidiaries	(9,852)	1,602	(10,361)	(4,843)
Net income attributable to Steel Dynamics, Inc.	31,550	72,303	62,311	110,882
Reportable legal entity | Guarantors
Condensed Consolidating Statements of Operations
Net sales	2,153,141	2,313,459	4,344,150	4,425,848
Costs of goods sold	2,019,025	2,183,181	4,091,150	4,198,757
Gross profit	134,116	130,278	253,000	227,091
Selling, general and administrative	65,736	56,411	123,769	110,700
Operating income	68,380	73,867	129,231	116,391
Interest expense, net of capitalized interest	18,376	10,294	40,216	20,855
Other (income) expense, net	(654)	(46)	34	349
Income before income taxes	50,658	63,619	88,981	95,187
Income taxes (benefit)	19,172	23,076	32,206	34,018
Net income	31,486	40,543	56,775	61,169
Net income attributable to Steel Dynamics, Inc.	31,486	40,543	56,775	61,169
Reportable legal entity | Combined Non-Guarantors
Condensed Consolidating Statements of Operations
Net sales	101,880	121,239	206,453	227,088
Costs of goods sold	134,335	141,362	253,116	264,935
Gross profit	(32,455)	(20,123)	(46,663)	(37,847)
Selling, general and administrative	2,668	3,406	6,033	6,814
Operating income	(35,123)	(23,529)	(52,696)	(44,661)
Interest expense, net of capitalized interest	1,658	1,986	3,392	3,866
Other (income) expense, net	(822)	(1,412)	(1,994)	(2,696)
Income before income taxes	(35,959)	(24,103)	(54,094)	(45,831)
Income taxes (benefit)	(2,510)	814	(3,626)	1,510
Net income	(33,449)	(24,917)	(50,468)	(47,341)
Net loss attributable to noncontrolling interests	6,225	5,962	10,032	10,843
Net income attributable to Steel Dynamics, Inc.	(27,224)	(18,955)	(40,436)	(36,498)
Consolidating Adjustments
Condensed Consolidating Statements of Operations
Net sales	(1,016,070)	(1,372,506)	(2,062,935)	(2,624,225)
Costs of goods sold	(989,355)	(1,337,577)	(2,013,443)	(2,571,343)
Gross profit	(26,715)	(34,929)	(49,492)	(52,882)
Selling, general and administrative	(4,125)	(4,370)	(8,995)	(8,529)
Operating income	(22,590)	(30,559)	(40,497)	(44,353)
Interest expense, net of capitalized interest	(1,037)	(1,261)	(2,061)	(2,494)
Other (income) expense, net	1,037	1,260	2,061	2,494
Income before income taxes	(22,590)	(30,558)	(40,497)	(44,353)
Income taxes (benefit)	(8,476)	(10,572)	(13,797)	(14,839)
Net income	(14,114)	(19,986)	(26,700)	(29,514)
Equity in net loss of subsidiaries	9,852	(1,602)	10,361	4,843
Net income attributable to Steel Dynamics, Inc.	$ (4,262)	$ (21,588)	$ (16,339)	$ (24,671)