Condensed Consolidating Information - Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended					9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Condensed Consolidating Statements of Operations
Net sales	$ 2,526,845			$ 3,223,547		$ 8,114,795	$ 8,917,947
Costs of goods sold	2,167,006			2,537,466		6,900,220	7,116,368
Gross profit	359,839			686,081		1,214,575	1,801,579
Selling, general and administrative	131,794			154,509		409,656	444,723
Operating income	228,045			531,572		804,919	1,356,856
Interest expense, net of capitalized interest	31,339			31,560		94,782	94,968
Other income, net	(4,545)			(7,103)		(15,137)	(16,601)
Income before income taxes	201,251			507,115		725,274	1,278,489
Income taxes	48,643			109,209		171,093	292,536
Net income	152,608	$ 196,746	$ 204,827	397,906	$ 362,572	554,181	985,953
Net (income) loss attributable to noncontrolling interests	(1,560)			469		(4,503)	2,422
Net income attributable to Steel Dynamics, Inc.	151,048			398,375		549,678	988,375
Parent
Condensed Consolidating Statements of Operations
Net sales	976,470			1,304,801		3,198,215	3,635,571
Costs of goods sold	814,622			956,735		2,628,849	2,746,516
Gross profit	161,848			348,066		569,366	889,055
Selling, general and administrative	55,613			78,824		185,647	218,839
Operating income	106,235			269,242		383,719	670,216
Interest expense, net of capitalized interest	18,716			18,559		56,818	56,568
Other income, net	(5,394)			(7,121)		(17,752)	(18,299)
Income before income taxes	92,913			257,804		344,653	631,947
Income taxes	23,625			45,599		81,422	127,083
Net income	69,288			212,205		263,231	504,864
Equity in net income of subsidiaries	81,760			186,170		286,447	483,511
Net income attributable to Steel Dynamics, Inc.	151,048			398,375		549,678	988,375
Guarantors
Condensed Consolidating Statements of Operations
Net sales	2,710,767			3,540,381		8,837,385	9,804,316
Costs of goods sold	2,502,257			3,163,110		8,141,120	8,783,145
Gross profit	208,510			377,271		696,265	1,021,171
Selling, general and administrative	74,433			78,694		221,592	233,989
Operating income	134,077			298,577		474,673	787,182
Interest expense, net of capitalized interest	11,536			12,489		35,107	36,793
Other income, net	(1,574)			(2,292)		(4,746)	(5,612)
Income before income taxes	124,115			288,380		444,312	756,001
Income taxes	29,965			71,466		107,384	187,794
Net income	94,150			216,914		336,928	568,207
Net income attributable to Steel Dynamics, Inc.	94,150			216,914		336,928	568,207
Combined Non-Guarantors
Condensed Consolidating Statements of Operations
Net sales	205,617			144,145		611,407	417,450
Costs of goods sold	188,967			142,005		568,914	413,607
Gross profit	16,650			2,140		42,493	3,843
Selling, general and administrative	7,827			2,974		19,891	8,550
Operating income	8,823			(834)		22,602	(4,707)
Interest expense, net of capitalized interest	3,490			3,134		10,327	9,489
Other income, net	(196)			(306)		(753)	(565)
Income before income taxes	5,529			(3,662)		13,028	(13,631)
Income taxes	128			420		495	530
Net income	5,401			(4,082)		12,533	(14,161)
Net (income) loss attributable to noncontrolling interests	(1,560)			469		(4,503)	2,422
Net income attributable to Steel Dynamics, Inc.	3,841			(3,613)		8,030	(11,739)
Consolidating Adjustments
Condensed Consolidating Statements of Operations
Net sales	(1,366,009)			(1,765,780)		(4,532,212)	(4,939,390)
Costs of goods sold	(1,338,840)			(1,724,384)		(4,438,663)	(4,826,900)
Gross profit	(27,169)			(41,396)		(93,549)	(112,490)
Selling, general and administrative	(6,079)			(5,983)		(17,474)	(16,655)
Operating income	(21,090)			(35,413)		(76,075)	(95,835)
Interest expense, net of capitalized interest	(2,403)			(2,622)		(7,470)	(7,882)
Other income, net	2,619			2,616		8,114	7,875
Income before income taxes	(21,306)			(35,407)		(76,719)	(95,828)
Income taxes	(5,075)			(8,276)		(18,208)	(22,871)
Net income	(16,231)			(27,131)		(58,511)	(72,957)
Equity in net income of subsidiaries	(81,760)			(186,170)		(286,447)	(483,511)
Net income attributable to Steel Dynamics, Inc.	$ (97,991)			$ (213,301)		$ (344,958)	$ (556,468)