Condensed Consolidating Information - Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Consolidating Statements of Operations
Net sales	$ 2,903,892	$ 3,223,547	$ 3,090,525	$ 2,603,875	$ 2,336,479	$ 2,443,382	$ 2,390,720	$ 2,368,216	$ 11,821,839	$ 9,538,797	$ 7,777,109
Costs of goods sold									9,499,025	7,956,783	6,442,245
Gross profit	521,235	686,081	652,082	463,416	320,824	396,518	392,518	472,154	2,322,814	1,582,014	1,334,864
Selling, general and administrative									600,405	515,133	474,059
Asset impairment charges											132,839
Operating income	365,553	531,572	501,887	323,397	195,947	271,015	265,353	334,566	1,722,409	1,066,881	727,966
Interest expense, net of capitalized interest									126,620	134,399	146,037
Other (income) expense, net									(23,985)	(2,753)	17,796
Income before income taxes									1,619,774	935,235	564,133
Income taxes (benefit)									363,969	129,439	204,127
Net income	269,852	397,906	362,572	225,475	303,171	151,012	152,947	198,666	1,255,805	805,796	360,006
Net loss attributable to noncontrolling interests									2,574	6,945	22,109
Net income attributable to Steel Dynamics, Inc.	$ 270,004	$ 398,375	$ 362,449	$ 227,551	$ 304,733	$ 153,258	$ 153,933	$ 200,817	1,258,379	812,741	382,115
Consolidating Adjustments
Condensed Consolidating Statements of Operations
Net sales									(6,553,114)	(5,128,115)	(4,005,490)
Costs of goods sold									(6,406,058)	(5,014,675)	(3,899,722)
Gross profit									(147,056)	(113,440)	(105,768)
Selling, general and administrative									(22,305)	(20,257)	(19,557)
Asset impairment charges											(1,806)
Operating income									(124,751)	(93,183)	(84,405)
Interest expense, net of capitalized interest									(10,492)	(9,645)	(8,439)
Other (income) expense, net									10,527	9,645	8,439
Income before income taxes									(124,786)	(93,183)	(84,405)
Income taxes (benefit)									(29,404)	(32,193)	(30,492)
Net income									(95,382)	(60,990)	(53,913)
Equity in net income of subsidiaries									(614,681)	(327,392)	(137,155)
Net income attributable to Steel Dynamics, Inc.									(710,063)	(388,382)	(191,068)
Parent Company [Member]
Condensed Consolidating Statements of Operations
Net sales									4,819,137	3,702,597	3,037,801
Costs of goods sold									3,675,766	2,994,257	2,428,863
Gross profit									1,143,371	708,340	608,938
Selling, general and administrative									298,101	221,736	196,442
Asset impairment charges											12,700
Operating income									845,270	486,604	399,796
Interest expense, net of capitalized interest									75,312	73,521	71,182
Other (income) expense, net									(27,268)	(2,305)	14,200
Income before income taxes									797,226	415,388	314,414
Income taxes (benefit)									153,528	(69,961)	69,454
Net income									643,698	485,349	244,960
Equity in net income of subsidiaries									614,681	327,392	137,155
Net income attributable to Steel Dynamics, Inc.									1,258,379	812,741	382,115
Guarantors
Condensed Consolidating Statements of Operations
Net sales									13,000,622	10,366,526	8,356,727
Costs of goods sold									11,680,691	9,399,996	7,522,791
Gross profit									1,319,931	966,530	833,936
Selling, general and administrative									312,483	293,161	281,071
Asset impairment charges											5,500
Operating income									1,007,448	673,369	547,365
Interest expense, net of capitalized interest									49,205	57,622	72,420
Other (income) expense, net									(6,514)	(9,937)	(5,363)
Income before income taxes									964,757	625,684	480,308
Income taxes (benefit)									239,198	226,327	180,990
Net income									725,559	399,357	299,318
Net income attributable to Steel Dynamics, Inc.									725,559	399,357	299,318
Combined Non-Guarantors
Condensed Consolidating Statements of Operations
Net sales									555,194	597,789	388,071
Costs of goods sold									548,626	577,205	390,313
Gross profit									6,568	20,584	(2,242)
Selling, general and administrative									12,126	20,493	16,103
Asset impairment charges											116,445
Operating income									(5,558)	91	(134,790)
Interest expense, net of capitalized interest									12,595	12,901	10,874
Other (income) expense, net									(730)	(156)	520
Income before income taxes									(17,423)	(12,654)	(146,184)
Income taxes (benefit)									647	5,266	(15,825)
Net income									(18,070)	(17,920)	(130,359)
Net loss attributable to noncontrolling interests									2,574	6,945	22,109
Net income attributable to Steel Dynamics, Inc.									$ (15,496)	$ (10,975)	$ (108,250)