Condensed Consolidating Information - Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Condensed Consolidating Statements of Operations
Net sales	$ 2,770,515		$ 3,090,525	$ 5,587,950	$ 5,694,400
Costs of goods sold	2,349,349		2,438,443	4,733,214	4,578,902
Gross profit	421,166		652,082	854,736	1,115,498
Selling, general and administrative	136,134		150,195	277,862	290,214
Operating income	285,032		501,887	576,874	825,284
Interest expense, net of capitalized interest	32,321		31,512	63,443	63,408
Other (income) expense, net	(4,249)		(5,035)	(10,592)	(9,498)
Income before income taxes	256,960		475,410	524,023	771,374
Income taxes (benefit)	60,214		112,838	122,450	183,327
Net income	196,746	$ 204,827	362,572	401,573	588,047
Net (income) loss attributable to noncontrolling interests	(2,444)		(123)	(2,943)	1,953
Net income attributable to Steel Dynamics, Inc.	194,302		362,449	398,630	590,000
Parent
Condensed Consolidating Statements of Operations
Net sales	1,097,010		1,294,096	2,221,745	2,330,770
Costs of goods sold	908,232		970,636	1,814,227	1,789,781
Gross profit	188,778		323,460	407,518	540,989
Selling, general and administrative	61,028		77,088	130,034	140,015
Operating income	127,750		246,372	277,484	400,974
Interest expense, net of capitalized interest	19,448		19,386	38,102	38,009
Other (income) expense, net	(4,958)		(5,975)	(12,358)	(11,178)
Income before income taxes	113,260		232,961	251,740	374,143
Income taxes (benefit)	27,033		51,738	57,797	81,484
Net income	86,227		181,223	193,943	292,659
Equity in net income of subsidiaries	108,075		181,226	204,687	297,341
Net income attributable to Steel Dynamics, Inc.	194,302		362,449	398,630	590,000
Guarantors
Condensed Consolidating Statements of Operations
Net sales	3,016,389		3,405,216	6,126,618	6,263,935
Costs of goods sold	2,772,874		3,038,284	5,638,863	5,620,035
Gross profit	243,515		366,932	487,755	643,900
Selling, general and administrative	72,576		75,923	147,159	155,295
Operating income	170,939		291,009	340,596	488,605
Interest expense, net of capitalized interest	11,805		11,593	23,571	24,304
Other (income) expense, net	(1,795)		(1,595)	(3,172)	(3,320)
Income before income taxes	160,929		281,011	320,197	467,621
Income taxes (benefit)	38,885		69,910	77,419	116,328
Net income	122,044		211,101	242,778	351,293
Net income attributable to Steel Dynamics, Inc.	122,044		211,101	242,778	351,293
Combined Non-Guarantors
Condensed Consolidating Statements of Operations
Net sales	233,907		156,308	405,790	273,305
Costs of goods sold	214,487		153,104	379,947	271,602
Gross profit	19,420		3,204	25,843	1,703
Selling, general and administrative	8,134		2,400	12,064	5,576
Operating income	11,286		804	13,779	(3,873)
Interest expense, net of capitalized interest	3,623		3,089	6,837	6,355
Other (income) expense, net	(265)		(13)	(557)	(259)
Income before income taxes	7,928		(2,272)	7,499	(9,969)
Income taxes (benefit)	264		(101)	367	110
Net income	7,664		(2,171)	7,132	(10,079)
Net (income) loss attributable to noncontrolling interests	(2,444)		(123)	(2,943)	1,953
Net income attributable to Steel Dynamics, Inc.	5,220		(2,294)	4,189	(8,126)
Consolidating Adjustments
Condensed Consolidating Statements of Operations
Net sales	(1,576,791)		(1,765,095)	(3,166,203)	(3,173,610)
Costs of goods sold	(1,546,244)		(1,723,581)	(3,099,823)	(3,102,516)
Gross profit	(30,547)		(41,514)	(66,380)	(71,094)
Selling, general and administrative	(5,604)		(5,216)	(11,395)	(10,672)
Operating income	(24,943)		(36,298)	(54,985)	(60,422)
Interest expense, net of capitalized interest	(2,555)		(2,556)	(5,067)	(5,260)
Other (income) expense, net	2,769		2,548	5,495	5,259
Income before income taxes	(25,157)		(36,290)	(55,413)	(60,421)
Income taxes (benefit)	(5,968)		(8,709)	(13,133)	(14,595)
Net income	(19,189)		(27,581)	(42,280)	(45,826)
Equity in net income of subsidiaries	(108,075)		(181,226)	(204,687)	(297,341)
Net income attributable to Steel Dynamics, Inc.	$ (127,264)		$ (208,807)	$ (246,967)	$ (343,167)