Condensed Consolidating Information - Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Condensed Consolidating Statements of Operations
Net sales	$ 2,368,216	$ 1,741,301
Costs of goods sold	1,896,062	1,505,265
Gross profit	472,154	236,036
Selling, general and administrative	137,588	104,071
Operating income	334,566	131,965
Interest expense, net of capitalized interest	33,973	37,043
Other (income) expense, net	(3,659)	(1,792)
Income before income taxes	304,252	96,714
Income taxes (benefit)	105,586	35,396
Net income	198,666	61,318
Net loss attributable to noncontrolling interests	2,151	1,419
Net income attributable to Steel Dynamics, Inc.	200,817	62,737
Parent
Condensed Consolidating Statements of Operations
Net sales	924,566	657,814
Costs of goods sold	711,579	548,179
Gross profit	212,987	109,635
Selling, general and administrative	61,655	38,602
Operating income	151,332	71,033
Interest expense, net of capitalized interest	18,081	18,187
Other (income) expense, net	(3,254)	(2,208)
Income before income taxes	136,505	55,054
Income taxes (benefit)	41,585	17,268
Net income	94,920	37,786
Equity in net income of subsidiaries	105,897	24,951
Net income attributable to Steel Dynamics, Inc.	200,817	62,737
Guarantors
Condensed Consolidating Statements of Operations
Net sales	2,572,238	1,857,344
Costs of goods sold	2,289,664	1,701,821
Gross profit	282,574	155,523
Selling, general and administrative	76,282	67,200
Operating income	206,292	88,323
Interest expense, net of capitalized interest	15,123	18,400
Other (income) expense, net	(2,667)	2,190
Income before income taxes	193,836	67,733
Income taxes (benefit)	70,311	25,495
Net income	123,525	42,238
Net income attributable to Steel Dynamics, Inc.	123,525	42,238
Combined Non-Guarantors
Condensed Consolidating Statements of Operations
Net sales	143,408	85,740
Costs of goods sold	138,041	91,030
Gross profit	5,367	(5,290)
Selling, general and administrative	5,053	2,802
Operating income	314	(8,092)
Interest expense, net of capitalized interest	3,266	2,260
Other (income) expense, net	(236)	(3,578)
Income before income taxes	(2,716)	(6,774)
Income taxes (benefit)	1,790	(271)
Net income	(4,506)	(6,503)
Net loss attributable to noncontrolling interests	2,151	1,419
Net income attributable to Steel Dynamics, Inc.	(2,355)	(5,084)
Consolidating Adjustments
Condensed Consolidating Statements of Operations
Net sales	(1,271,996)	(859,597)
Costs of goods sold	(1,243,222)	(835,765)
Gross profit	(28,774)	(23,832)
Selling, general and administrative	(5,402)	(4,533)
Operating income	(23,372)	(19,299)
Interest expense, net of capitalized interest	(2,497)	(1,804)
Other (income) expense, net	2,498	1,804
Income before income taxes	(23,373)	(19,299)
Income taxes (benefit)	(8,100)	(7,096)
Net income	(15,273)	(12,203)
Equity in net income of subsidiaries	(105,897)	(24,951)
Net income attributable to Steel Dynamics, Inc.	$ (121,170)	$ (37,154)