Condensed Consolidating Information - Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Condensed Consolidating Statements of Operations
Net sales	$ 1,741,301	$ 2,047,435
Costs of goods sold	1,505,265	1,860,393
Gross profit	236,036	187,042
Selling, general and administrative	104,071	87,271
Operating income	131,965	99,771
Interest expense, net of capitalized interest	37,043	43,087
Other (income) expense, net	(1,792)	16,192
Income before income taxes	96,714	40,492
Income taxes (benefit)	35,396	13,538
Net income	61,318	26,954
Net loss attributable to noncontrolling interests	1,419	3,807
Net income attributable to Steel Dynamics, Inc.	62,737	30,761
Reportable legal entity | Parent
Condensed Consolidating Statements of Operations
Net sales	657,814	798,718
Costs of goods sold	548,179	693,575
Gross profit	109,635	105,143
Selling, general and administrative	38,602	30,743
Operating income	71,033	74,400
Interest expense, net of capitalized interest	18,187	20,537
Other (income) expense, net	(2,208)	15,652
Income before income taxes	55,054	38,211
Income taxes (benefit)	17,268	6,941
Net income	37,786	31,270
Equity in net loss of subsidiaries	24,951	(509)
Net income attributable to Steel Dynamics, Inc.	62,737	30,761
Reportable legal entity | Guarantors
Condensed Consolidating Statements of Operations
Net sales	1,857,344	2,191,009
Costs of goods sold	1,701,821	2,072,125
Gross profit	155,523	118,884
Selling, general and administrative	67,200	58,033
Operating income	88,323	60,851
Interest expense, net of capitalized interest	18,400	21,840
Other (income) expense, net	2,190	688
Income before income taxes	67,733	38,323
Income taxes (benefit)	25,495	13,034
Net income	42,238	25,289
Net income attributable to Steel Dynamics, Inc.	42,238	25,289
Reportable legal entity | Combined Non-Guarantors
Condensed Consolidating Statements of Operations
Net sales	85,740	104,573
Costs of goods sold	91,030	118,781
Gross profit	(5,290)	(14,208)
Selling, general and administrative	2,802	3,365
Operating income	(8,092)	(17,573)
Interest expense, net of capitalized interest	2,260	1,734
Other (income) expense, net	(3,578)	(1,172)
Income before income taxes	(6,774)	(18,135)
Income taxes (benefit)	(271)	(1,116)
Net income	(6,503)	(17,019)
Net loss attributable to noncontrolling interests	1,419	3,807
Net income attributable to Steel Dynamics, Inc.	(5,084)	(13,212)
Consolidating Adjustments
Condensed Consolidating Statements of Operations
Net sales	(859,597)	(1,046,865)
Costs of goods sold	(835,765)	(1,024,088)
Gross profit	(23,832)	(22,777)
Selling, general and administrative	(4,533)	(4,870)
Operating income	(19,299)	(17,907)
Interest expense, net of capitalized interest	(1,804)	(1,024)
Other (income) expense, net	1,804	1,024
Income before income taxes	(19,299)	(17,907)
Income taxes (benefit)	(7,096)	(5,321)
Net income	(12,203)	(12,586)
Equity in net loss of subsidiaries	(24,951)	509
Net income attributable to Steel Dynamics, Inc.	$ (37,154)	$ (12,077)