Condensed Consolidating Information (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Condensed Consolidating Statements of Operations
Net sales	$ 2,047,435	$ 1,830,082
Costs of goods sold	1,860,393	1,666,778
Gross profit	187,042	163,304
Selling, general and administrative	87,271	82,372
Operating income	99,771	80,932
Interest expense, net of capitalized interest	43,087	30,569
Other (income) expense, net	16,192	(631)
Income before income taxes	40,492	50,994
Income taxes (benefit)	13,538	17,296
Net income	26,954	33,698
Net loss attributable to noncontrolling interests	3,807	4,881
Net income attributable to Steel Dynamics, Inc.	30,761	38,579
Reportable legal entity | Parent
Condensed Consolidating Statements of Operations
Net sales	798,718	863,563
Costs of goods sold	693,575	761,395
Gross profit	105,143	102,168
Selling, general and administrative	30,743	28,834
Operating income	74,400	73,334
Interest expense, net of capitalized interest	20,537	19,361
Other (income) expense, net	15,652	(976)
Income before income taxes	38,211	54,949
Income taxes (benefit)	6,941	9,925
Net income	31,270	45,024
Equity in net loss of subsidiaries	(509)	(6,445)
Net income attributable to Steel Dynamics, Inc.	30,761	38,579
Reportable legal entity | Guarantors
Condensed Consolidating Statements of Operations
Net sales	2,191,009	2,112,389
Costs of goods sold	2,072,125	2,015,576
Gross profit	118,884	96,813
Selling, general and administrative	58,033	54,289
Operating income	60,851	42,524
Interest expense, net of capitalized interest	21,840	10,561
Other (income) expense, net	688	395
Income before income taxes	38,323	31,568
Income taxes (benefit)	13,034	10,942
Net income	25,289	20,626
Net income attributable to Steel Dynamics, Inc.	25,289	20,626
Reportable legal entity | Combined Non-Guarantors
Condensed Consolidating Statements of Operations
Net sales	104,573	105,849
Costs of goods sold	118,781	123,573
Gross profit	(14,208)	(17,724)
Selling, general and administrative	3,365	3,408
Operating income	(17,573)	(21,132)
Interest expense, net of capitalized interest	1,734	1,880
Other (income) expense, net	(1,172)	(1,284)
Income before income taxes	(18,135)	(21,728)
Income taxes (benefit)	(1,116)	696
Net income	(17,019)	(22,424)
Net loss attributable to noncontrolling interests	3,807	4,881
Net income attributable to Steel Dynamics, Inc.	(13,212)	(17,543)
Consolidating Adjustments
Condensed Consolidating Statements of Operations
Net sales	(1,046,865)	(1,251,719)
Costs of goods sold	(1,024,088)	(1,233,766)
Gross profit	(22,777)	(17,953)
Selling, general and administrative	(4,870)	(4,159)
Operating income	(17,907)	(13,794)
Interest expense, net of capitalized interest	(1,024)	(1,233)
Other (income) expense, net	1,024	1,234
Income before income taxes	(17,907)	(13,795)
Income taxes (benefit)	(5,321)	(4,267)
Net income	(12,586)	(9,528)
Equity in net loss of subsidiaries	509	6,445
Net income attributable to Steel Dynamics, Inc.	$ (12,077)	$ (3,083)