Condensed Consolidating Information - Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Condensed Consolidating Statements of Operations
Net sales	$ 2,603,875	$ 2,368,216
Costs of goods sold	2,140,459	1,896,062
Gross profit	463,416	472,154
Selling, general and administrative	140,019	137,588
Operating income	323,397	334,566
Interest expense, net of capitalized interest	31,896	33,973
Other (income) expense, net	(4,463)	(3,659)
Income before income taxes	295,964	304,252
Income taxes (benefit)	70,489	105,586
Net income	225,475	198,666
Net loss attributable to noncontrolling interests	2,076	2,151
Net income attributable to Steel Dynamics, Inc.	227,551	200,817
Parent
Condensed Consolidating Statements of Operations
Net sales	1,036,674	924,566
Costs of goods sold	819,145	711,579
Gross profit	217,529	212,987
Selling, general and administrative	62,927	61,655
Operating income	154,602	151,332
Interest expense, net of capitalized interest	18,623	18,081
Other (income) expense, net	(5,203)	(3,254)
Income before income taxes	141,182	136,505
Income taxes (benefit)	29,746	41,585
Net income	111,436	94,920
Equity in net income of subsidiaries	116,115	105,897
Net income attributable to Steel Dynamics, Inc.	227,551	200,817
Guarantors
Condensed Consolidating Statements of Operations
Net sales	2,823,129	2,572,238
Costs of goods sold	2,551,620	2,289,664
Gross profit	271,509	282,574
Selling, general and administrative	76,952	76,282
Operating income	194,557	206,292
Interest expense, net of capitalized interest	12,437	15,123
Other (income) expense, net	(1,697)	(2,667)
Income before income taxes	183,817	193,836
Income taxes (benefit)	45,720	70,311
Net income	138,097	123,525
Net income attributable to Steel Dynamics, Inc.	138,097	123,525
Combined Non-Guarantors
Condensed Consolidating Statements of Operations
Net sales	152,587	143,408
Costs of goods sold	148,630	138,041
Gross profit	3,957	5,367
Selling, general and administrative	5,597	5,053
Operating income	(1,640)	314
Interest expense, net of capitalized interest	3,540	3,266
Other (income) expense, net	(273)	(236)
Income before income taxes	(4,907)	(2,716)
Income taxes (benefit)	909	1,790
Net income	(5,816)	(4,506)
Net loss attributable to noncontrolling interests	2,076	2,151
Net income attributable to Steel Dynamics, Inc.	(3,740)	(2,355)
Consolidating Adjustments
Condensed Consolidating Statements of Operations
Net sales	(1,408,515)	(1,271,996)
Costs of goods sold	(1,378,936)	(1,243,222)
Gross profit	(29,579)	(28,774)
Selling, general and administrative	(5,457)	(5,402)
Operating income	(24,122)	(23,372)
Interest expense, net of capitalized interest	(2,704)	(2,497)
Other (income) expense, net	2,710	2,498
Income before income taxes	(24,128)	(23,373)
Income taxes (benefit)	(5,886)	(8,100)
Net income	(18,242)	(15,273)
Equity in net income of subsidiaries	(116,115)	(105,897)
Net income attributable to Steel Dynamics, Inc.	$ (134,357)	$ (121,170)