Condensed Consolidating Information - Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Condensed Consolidating Statements of Operations
Net sales	$ 2,817,435	$ 2,603,875
Costs of goods sold	2,383,865	2,140,459
Gross profit	433,570	463,416
Selling, general and administrative	141,728	140,019
Operating income	291,842	323,397
Interest expense, net of capitalized interest	31,122	31,896
Other (income) expense, net	(6,343)	(4,463)
Income before income taxes	267,063	295,964
Income taxes	62,236	70,489
Net income	204,827	225,475
Net (income) loss attributable to noncontrolling interests	(499)	2,076
Net income attributable to Steel Dynamics, Inc.	204,328	227,551
Parent
Condensed Consolidating Statements of Operations
Net sales	1,124,735	1,036,674
Costs of goods sold	905,995	819,145
Gross profit	218,740	217,529
Selling, general and administrative	69,006	62,927
Operating income	149,734	154,602
Interest expense, net of capitalized interest	18,654	18,623
Other (income) expense, net	(7,400)	(5,203)
Income before income taxes	138,480	141,182
Income taxes	30,764	29,746
Net income	107,716	111,436
Equity in net income of subsidiaries	96,612	116,115
Net income attributable to Steel Dynamics, Inc.	204,328	227,551
Guarantors
Condensed Consolidating Statements of Operations
Net sales	3,110,229	2,823,129
Costs of goods sold	2,865,989	2,551,620
Gross profit	244,240	271,509
Selling, general and administrative	74,583	76,952
Operating income	169,657	194,557
Interest expense, net of capitalized interest	11,766	12,437
Other (income) expense, net	(1,377)	(1,697)
Income before income taxes	159,268	183,817
Income taxes	38,534	45,720
Net income	120,734	138,097
Net income attributable to Steel Dynamics, Inc.	120,734	138,097
Combined Non-Guarantors
Condensed Consolidating Statements of Operations
Net sales	171,883	152,587
Costs of goods sold	165,460	148,630
Gross profit	6,423	3,957
Selling, general and administrative	3,930	5,597
Operating income	2,493	(1,640)
Interest expense, net of capitalized interest	3,214	3,540
Other (income) expense, net	(292)	(273)
Income before income taxes	(429)	(4,907)
Income taxes	103	909
Net income	(532)	(5,816)
Net (income) loss attributable to noncontrolling interests	(499)	2,076
Net income attributable to Steel Dynamics, Inc.	(1,031)	(3,740)
Consolidating Adjustments
Condensed Consolidating Statements of Operations
Net sales	(1,589,412)	(1,408,515)
Costs of goods sold	(1,553,579)	(1,378,936)
Gross profit	(35,833)	(29,579)
Selling, general and administrative	(5,791)	(5,457)
Operating income	(30,042)	(24,122)
Interest expense, net of capitalized interest	(2,512)	(2,704)
Other (income) expense, net	2,726	2,710
Income before income taxes	(30,256)	(24,128)
Income taxes	(7,165)	(5,886)
Net income	(23,091)	(18,242)
Equity in net income of subsidiaries	(96,612)	(116,115)
Net income attributable to Steel Dynamics, Inc.	$ (119,703)	$ (134,357)