Condensed Consolidating Information - Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Condensed Consolidating Statements of Operations
Net sales	$ 2,101,310	$ 1,950,923	$ 5,866,513	$ 6,003,365
Costs of goods sold	1,692,807	1,722,197	4,841,591	5,415,854
Gross profit	408,503	228,726	1,024,922	587,511
Selling, general and administrative	124,648	97,697	352,980	279,152
Operating income	283,855	131,029	671,942	308,359
Interest expense, net of capitalized interest	36,199	37,084	109,888	117,334
Other (income) expense, net	4,351	239	741	15,219
Income before income taxes	243,305	93,706	561,313	175,806
Income taxes (benefit)	88,892	34,839	205,139	64,660
Net income	154,413	58,867	356,174	111,146
Net loss attributable to noncontrolling interests	2,984	1,750	5,929	11,782
Net income attributable to Steel Dynamics, Inc.	157,397	60,617	362,103	122,928
Parent
Condensed Consolidating Statements of Operations
Net sales	845,585	735,250	2,297,389	2,300,024
Costs of goods sold	647,983	619,833	1,823,408	1,982,667
Gross profit	197,602	115,417	473,981	317,357
Selling, general and administrative	52,995	35,235	145,596	95,883
Operating income	144,607	80,182	328,385	221,474
Interest expense, net of capitalized interest	17,818	18,312	53,842	57,015
Other (income) expense, net	2,342	252	(2,137)	15,131
Income before income taxes	124,447	61,618	276,680	149,328
Income taxes (benefit)	40,242	20,169	89,210	35,207
Net income	84,205	41,449	187,470	114,121
Equity in net income of subsidiaries	73,192	19,168	174,633	8,807
Net income attributable to Steel Dynamics, Inc.	157,397	60,617	362,103	122,928
Guarantors
Condensed Consolidating Statements of Operations
Net sales	2,258,466	2,103,824	6,322,158	6,447,974
Costs of goods sold	2,016,874	1,965,364	5,676,265	6,056,514
Gross profit	241,592	138,460	645,893	391,460
Selling, general and administrative	72,605	64,259	212,613	188,028
Operating income	168,987	74,201	433,280	203,432
Interest expense, net of capitalized interest	17,741	18,138	54,493	58,354
Other (income) expense, net	3,937	(631)	8,012	(597)
Income before income taxes	147,309	56,694	370,775	145,675
Income taxes (benefit)	55,684	21,697	139,958	53,903
Net income	91,625	34,997	230,817	91,772
Net income attributable to Steel Dynamics, Inc.	91,625	34,997	230,817	91,772
Combined Non-Guarantors
Condensed Consolidating Statements of Operations
Net sales	96,409	96,204	271,694	302,657
Costs of goods sold	100,470	99,864	286,486	352,980
Gross profit	(4,061)	(3,660)	(14,792)	(50,323)
Selling, general and administrative	4,134	2,845	9,352	8,878
Operating income	(8,195)	(6,505)	(24,144)	(59,201)
Interest expense, net of capitalized interest	2,847	1,692	7,558	5,084
Other (income) expense, net	(4,135)	(440)	(11,139)	(2,434)
Income before income taxes	(6,907)	(7,757)	(20,563)	(61,851)
Income taxes (benefit)	797	(421)	(63)	(4,047)
Net income	(7,704)	(7,336)	(20,500)	(57,804)
Net loss attributable to noncontrolling interests	2,984	1,750	5,929	11,782
Net income attributable to Steel Dynamics, Inc.	(4,720)	(5,586)	(14,571)	(46,022)
Consolidating Adjustments
Condensed Consolidating Statements of Operations
Net sales	(1,099,150)	(984,355)	(3,024,728)	(3,047,290)
Costs of goods sold	(1,072,520)	(962,864)	(2,944,568)	(2,976,307)
Gross profit	(26,630)	(21,491)	(80,160)	(70,983)
Selling, general and administrative	(5,086)	(4,642)	(14,581)	(13,637)
Operating income	(21,544)	(16,849)	(65,579)	(57,346)
Interest expense, net of capitalized interest	(2,207)	(1,058)	(6,005)	(3,119)
Other (income) expense, net	2,207	1,058	6,005	3,119
Income before income taxes	(21,544)	(16,849)	(65,579)	(57,346)
Income taxes (benefit)	(7,831)	(6,606)	(23,966)	(20,403)
Net income	(13,713)	(10,243)	(41,613)	(36,943)
Equity in net income of subsidiaries	(73,192)	(19,168)	(174,633)	(8,807)
Net income attributable to Steel Dynamics, Inc.	$ (86,905)	$ (29,411)	$ (216,246)	$ (45,750)