Condensed Consolidating Information - Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Condensed Consolidating Statements of Operations
Net sales	$ 2,443,382	$ 2,101,310	$ 7,202,318	$ 5,866,513
Costs of goods sold	2,046,864	1,692,807	5,941,128	4,841,591
Gross profit	396,518	408,503	1,261,190	1,024,922
Selling, general and administrative	125,503	124,648	390,256	352,980
Operating income	271,015	283,855	870,934	671,942
Interest expense, net of capitalized interest	34,177	36,199	102,019	109,888
Other (income) expense, net	2,526	4,351	(4,968)	741
Income before income taxes	234,312	243,305	773,883	561,313
Income taxes (benefit)	83,300	88,892	271,258	205,139
Net income	151,012	154,413	502,625	356,174
Net loss attributable to noncontrolling interests	2,246	2,984	5,383	5,929
Net income attributable to Steel Dynamics, Inc.	153,258	157,397	508,008	362,103
Parent
Condensed Consolidating Statements of Operations
Net sales	946,136	845,585	2,797,557	2,297,389
Costs of goods sold	763,310	647,983	2,241,936	1,823,408
Gross profit	182,826	197,602	555,621	473,981
Selling, general and administrative	51,464	52,995	164,061	145,596
Operating income	131,362	144,607	391,560	328,385
Interest expense, net of capitalized interest	18,831	17,818	55,353	53,842
Other (income) expense, net	2,660	2,342	(4,463)	(2,137)
Income before income taxes	109,871	124,447	340,670	276,680
Income taxes (benefit)	35,309	40,242	106,036	89,210
Net income	74,562	84,205	234,634	187,470
Equity in net income of subsidiaries	78,696	73,192	273,374	174,633
Net income attributable to Steel Dynamics, Inc.	153,258	157,397	508,008	362,103
Guarantors
Condensed Consolidating Statements of Operations
Net sales	2,641,067	2,258,466	7,827,981	6,322,158
Costs of goods sold	2,402,670	2,016,874	7,054,938	5,676,265
Gross profit	238,397	241,592	773,043	645,893
Selling, general and administrative	74,151	72,605	226,153	212,613
Operating income	164,246	168,987	546,890	433,280
Interest expense, net of capitalized interest	14,481	17,741	44,245	54,493
Other (income) expense, net	(2,492)	3,937	(7,305)	8,012
Income before income taxes	152,257	147,309	509,950	370,775
Income taxes (benefit)	55,183	55,684	184,891	139,958
Net income	97,074	91,625	325,059	230,817
Net income attributable to Steel Dynamics, Inc.	97,074	91,625	325,059	230,817
Combined Non-Guarantors
Condensed Consolidating Statements of Operations
Net sales	161,556	96,409	449,902	271,694
Costs of goods sold	157,564	100,470	432,657	286,486
Gross profit	3,992	(4,061)	17,245	(14,792)
Selling, general and administrative	4,896	4,134	15,253	9,352
Operating income	(904)	(8,195)	1,992	(24,144)
Interest expense, net of capitalized interest	3,181	2,847	9,494	7,558
Other (income) expense, net	42	(4,135)	(273)	(11,139)
Income before income taxes	(4,127)	(6,907)	(7,229)	(20,563)
Income taxes (benefit)	1,169	797	4,651	(63)
Net income	(5,296)	(7,704)	(11,880)	(20,500)
Net loss attributable to noncontrolling interests	2,246	2,984	5,383	5,929
Net income attributable to Steel Dynamics, Inc.	(3,050)	(4,720)	(6,497)	(14,571)
Consolidating Adjustments
Condensed Consolidating Statements of Operations
Net sales	(1,305,377)	(1,099,150)	(3,873,122)	(3,024,728)
Costs of goods sold	(1,276,680)	(1,072,520)	(3,788,403)	(2,944,568)
Gross profit	(28,697)	(26,630)	(84,719)	(80,160)
Selling, general and administrative	(5,008)	(5,086)	(15,211)	(14,581)
Operating income	(23,689)	(21,544)	(69,508)	(65,579)
Interest expense, net of capitalized interest	(2,316)	(2,207)	(7,073)	(6,005)
Other (income) expense, net	2,316	2,207	7,073	6,005
Income before income taxes	(23,689)	(21,544)	(69,508)	(65,579)
Income taxes (benefit)	(8,361)	(7,831)	(24,320)	(23,966)
Net income	(15,328)	(13,713)	(45,188)	(41,613)
Equity in net income of subsidiaries	(78,696)	(73,192)	(273,374)	(174,633)
Net income attributable to Steel Dynamics, Inc.	$ (94,024)	$ (86,905)	$ (318,562)	$ (216,246)