Condensed Consolidating Information - Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Consolidating Statements of Operations
Net sales	$ 2,336,479	$ 2,443,382	$ 2,390,720	$ 2,368,216	$ 1,910,596	$ 2,101,310	$ 2,023,902	$ 1,741,301	$ 9,538,797	$ 7,777,109	$ 7,594,411
Costs of goods sold									7,956,783	6,442,245	6,862,693
Gross profit	320,824	396,518	392,518	472,154	309,942	408,503	380,383	236,036	1,582,014	1,334,864	731,718
Selling, general and administrative									515,133	474,059	376,002
Asset impairment charges										132,839	428,500
Operating income	195,947	271,015	265,353	334,566	56,024	283,855	256,122	131,965	1,066,881	727,966	(72,784)
Interest expense, net of capitalized interest									134,399	146,037	153,950
Other (income) expense, net									(2,753)	17,796	15,383
Income before income taxes									935,235	564,133	(242,117)
Income taxes (benefit)									129,439	204,127	(96,947)
Net income	303,171	151,012	152,947	198,666	3,832	154,413	140,443	61,318	805,796	360,006	(145,170)
Net loss attributable to noncontrolling interests									6,945	22,109	14,859
Net income attributable to Steel Dynamics, Inc.	$ 304,733	$ 153,258	$ 153,933	$ 200,817	$ 20,012	$ 157,397	$ 141,969	$ 62,737	812,741	382,115	(130,311)
Parent
Condensed Consolidating Statements of Operations
Net sales									3,702,597	3,037,801	2,908,904
Costs of goods sold									2,994,257	2,428,863	2,515,621
Gross profit									708,340	608,938	393,283
Selling, general and administrative									221,736	196,442	128,929
Asset impairment charges										12,700
Operating income									486,604	399,796	264,354
Interest expense, net of capitalized interest									73,521	71,182	75,457
Other (income) expense, net									(2,305)	14,200	13,970
Income before income taxes									415,388	314,414	174,927
Income taxes (benefit)									(69,961)	69,454	(124,626)
Net income									485,349	244,960	299,553
Equity in net income of subsidiaries									327,392	137,155	(429,864)
Net income attributable to Steel Dynamics, Inc.									812,741	382,115	(130,311)
Guarantors
Condensed Consolidating Statements of Operations
Net sales									10,366,526	8,356,727	8,132,043
Costs of goods sold									9,399,996	7,522,791	7,646,411
Gross profit									966,530	833,936	485,632
Selling, general and administrative									293,161	281,071	254,106
Asset impairment charges										5,500	428,500
Operating income									673,369	547,365	(196,974)
Interest expense, net of capitalized interest									57,622	72,420	76,009
Other (income) expense, net									(9,937)	(5,363)	332
Income before income taxes									625,684	480,308	(273,315)
Income taxes (benefit)									226,327	180,990	61,571
Net income									399,357	299,318	(334,886)
Net income attributable to Steel Dynamics, Inc.									399,357	299,318	(334,886)
Combined Non-Guarantors
Condensed Consolidating Statements of Operations
Net sales									597,789	388,071	361,944
Costs of goods sold									577,205	390,313	420,281
Gross profit									20,584	(2,242)	(58,337)
Selling, general and administrative									20,493	16,103	11,893
Asset impairment charges										116,445
Operating income									91	(134,790)	(70,230)
Interest expense, net of capitalized interest									12,901	10,874	6,969
Other (income) expense, net									(156)	520	(3,404)
Income before income taxes									(12,654)	(146,184)	(73,795)
Income taxes (benefit)									5,266	(15,825)	(4,627)
Net income									(17,920)	(130,359)	(69,168)
Net loss attributable to noncontrolling interests									6,945	22,109	14,859
Net income attributable to Steel Dynamics, Inc.									(10,975)	(108,250)	(54,309)
Consolidating Adjustments
Condensed Consolidating Statements of Operations
Net sales									(5,128,115)	(4,005,490)	(3,808,480)
Costs of goods sold									(5,014,675)	(3,899,722)	(3,719,620)
Gross profit									(113,440)	(105,768)	(88,860)
Selling, general and administrative									(20,257)	(19,557)	(18,926)
Asset impairment charges										(1,806)
Operating income									(93,183)	(84,405)	(69,934)
Interest expense, net of capitalized interest									(9,645)	(8,439)	(4,485)
Other (income) expense, net									9,645	8,439	4,485
Income before income taxes									(93,183)	(84,405)	(69,934)
Income taxes (benefit)									(32,193)	(30,492)	(29,265)
Net income									(60,990)	(53,913)	(40,669)
Equity in net income of subsidiaries									(327,392)	(137,155)	429,864
Net income attributable to Steel Dynamics, Inc.									$ (388,382)	$ (191,068)	$ 389,195