Condensed Consolidating Information - Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Condensed Consolidating Statements of Operations
Net sales	$ 2,023,902	$ 2,005,007	$ 3,765,203	$ 4,052,442
Costs of goods sold	1,643,519	1,833,264	3,148,784	3,693,657
Gross profit	380,383	171,743	616,419	358,785
Selling, general and administrative	124,261	94,184	228,332	181,455
Operating income	256,122	77,559	388,087	177,330
Interest expense, net of capitalized interest	36,646	37,163	73,689	80,250
Other (income) expense, net	(1,818)	(1,212)	(3,610)	14,980
Income before income taxes	221,294	41,608	318,008	82,100
Income taxes (benefit)	80,851	16,283	116,247	29,821
Net income	140,443	25,325	201,761	52,279
Net loss attributable to noncontrolling interests	1,526	6,225	2,945	10,032
Net income attributable to Steel Dynamics, Inc.	141,969	31,550	204,706	62,311
Reportable legal entity | Parent
Condensed Consolidating Statements of Operations
Net sales	793,990	766,056	1,451,804	1,564,774
Costs of goods sold	627,246	669,259	1,175,425	1,362,834
Gross profit	166,744	96,797	276,379	201,940
Selling, general and administrative	53,999	29,905	92,601	60,648
Operating income	112,745	66,892	183,778	141,292
Interest expense, net of capitalized interest	17,837	18,166	36,024	38,703
Other (income) expense, net	(2,271)	(773)	(4,479)	14,879
Income before income taxes	97,179	49,499	152,233	87,710
Income taxes (benefit)	31,700	8,097	48,968	15,038
Net income	65,479	41,402	103,265	72,672
Equity in net income of subsidiaries	76,490	(9,852)	101,441	(10,361)
Net income attributable to Steel Dynamics, Inc.	141,969	31,550	204,706	62,311
Reportable legal entity | Guarantors
Condensed Consolidating Statements of Operations
Net sales	2,206,348	2,153,141	4,063,692	4,344,150
Costs of goods sold	1,957,570	2,019,025	3,659,391	4,091,150
Gross profit	248,778	134,116	404,301	253,000
Selling, general and administrative	72,808	65,736	140,008	123,769
Operating income	175,970	68,380	264,293	129,231
Interest expense, net of capitalized interest	18,352	18,376	36,752	40,216
Other (income) expense, net	1,885	(654)	4,075	34
Income before income taxes	155,733	50,658	223,466	88,981
Income taxes (benefit)	58,779	19,172	84,274	32,206
Net income	96,954	31,486	139,192	56,775
Net income attributable to Steel Dynamics, Inc.	96,954	31,486	139,192	56,775
Reportable legal entity | Combined Non-Guarantors
Condensed Consolidating Statements of Operations
Net sales	89,545	101,880	175,285	206,453
Costs of goods sold	94,986	134,335	186,016	253,116
Gross profit	(5,441)	(32,455)	(10,731)	(46,663)
Selling, general and administrative	2,416	2,668	5,218	6,033
Operating income	(7,857)	(35,123)	(15,949)	(52,696)
Interest expense, net of capitalized interest	2,451	1,658	4,711	3,392
Other (income) expense, net	(3,426)	(822)	(7,004)	(1,994)
Income before income taxes	(6,882)	(35,959)	(13,656)	(54,094)
Income taxes (benefit)	(589)	(2,510)	(860)	(3,626)
Net income	(6,293)	(33,449)	(12,796)	(50,468)
Net loss attributable to noncontrolling interests	1,526	6,225	2,945	10,032
Net income attributable to Steel Dynamics, Inc.	(4,767)	(27,224)	(9,851)	(40,436)
Consolidating Adjustments
Condensed Consolidating Statements of Operations
Net sales	(1,065,981)	(1,016,070)	(1,925,578)	(2,062,935)
Costs of goods sold	(1,036,283)	(989,355)	(1,872,048)	(2,013,443)
Gross profit	(29,698)	(26,715)	(53,530)	(49,492)
Selling, general and administrative	(4,962)	(4,125)	(9,495)	(8,995)
Operating income	(24,736)	(22,590)	(44,035)	(40,497)
Interest expense, net of capitalized interest	(1,994)	(1,037)	(3,798)	(2,061)
Other (income) expense, net	1,994	1,037	3,798	2,061
Income before income taxes	(24,736)	(22,590)	(44,035)	(40,497)
Income taxes (benefit)	(9,039)	(8,476)	(16,135)	(13,797)
Net income	(15,697)	(14,114)	(27,900)	(26,700)
Equity in net income of subsidiaries	(76,490)	9,852	(101,441)	10,361
Net income attributable to Steel Dynamics, Inc.	$ (92,187)	$ (4,262)	$ (129,341)	$ (16,339)