Condensed Consolidating Information (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Consolidating Statements of Operations
Net sales	$ 2,517,093	$ 2,339,016	$ 2,069,761	$ 1,830,082	$ 1,864,150	$ 1,911,738	$ 1,801,340	$ 1,795,696	$ 8,755,952	$ 7,372,924	$ 7,290,234
Costs of goods sold									7,789,741	6,653,780	6,570,336
Gross profit	291,624	288,512	222,771	163,304	197,996	197,192	147,692	176,264	966,211	719,144	719,898
Selling, general and administrative									645,891	332,619	328,733
Operating income	(81,160)	188,643	131,905	80,932	107,822	113,273	69,198	96,232	320,320	386,525	391,165
Interest expense, net of capitalized interest									137,263	127,728	158,585
Other (income) expense, net									18,254	(4,033)	28,514
Income before income taxes									164,803	262,830	204,066
Income taxes (benefit)									73,153	99,314	61,785
Net income	(96,046)	87,657	66,341	33,698	47,901	51,090	23,273	41,252	91,650	163,516	142,281
Net loss attributable to noncontrolling interests									65,374	25,798	21,270
Net income attributable to Steel Dynamics, Inc.	(45,031)	91,173	72,303	38,579	54,655	57,486	28,958	48,215	157,024	189,314	163,551
Reportable legal entity | Parent
Condensed Consolidating Statements of Operations
Net sales									3,904,488	3,466,691	3,431,978
Costs of goods sold									3,305,269	2,981,591	2,977,756
Gross profit									599,219	485,100	454,222
Selling, general and administrative									149,374	118,975	102,142
Operating income									449,845	366,125	352,080
Interest expense, net of capitalized interest									78,442	81,361	104,058
Other (income) expense, net									18,976	(7,358)	28,677
Income before income taxes									352,427	292,122	219,345
Income taxes (benefit)									14,064	63,670	26,611
Net income									338,363	228,452	192,734
Equity in net loss of subsidiaries									(181,339)	(39,138)	(29,183)
Net income attributable to Steel Dynamics, Inc.									157,024	189,314	163,551
Reportable legal entity | Guarantors
Condensed Consolidating Statements of Operations
Net sales									9,761,236	8,219,499	8,322,995
Costs of goods sold									9,212,977	7,805,367	7,906,392
Gross profit									548,259	414,132	416,603
Selling, general and administrative									241,163	219,531	218,515
Operating income									307,096	194,601	198,088
Interest expense, net of capitalized interest									56,006	43,879	53,186
Other (income) expense, net									(23)	3,401	(1,531)
Income before income taxes									251,113	147,321	146,433
Income taxes (benefit)									96,315	54,448	51,011
Net income									154,798	92,873	95,422
Net income attributable to Steel Dynamics, Inc.									154,798	92,873	95,422
Reportable legal entity | Combined Non-Guarantors
Condensed Consolidating Statements of Operations
Net sales									527,584	347,288	177,527
Costs of goods sold									598,428	440,064	257,604
Gross profit									(70,844)	(92,776)	(80,077)
Selling, general and administrative									273,503	10,548	24,081
Operating income									(344,347)	(103,324)	(104,158)
Interest expense, net of capitalized interest									7,745	7,259	6,630
Other (income) expense, net									(5,629)	(4,847)	(3,921)
Income before income taxes									(346,463)	(105,736)	(106,867)
Income taxes (benefit)									(1,095)	6,406	247
Net income									(345,368)	(112,142)	(107,114)
Net loss attributable to noncontrolling interests									65,374	25,798	21,270
Net income attributable to Steel Dynamics, Inc.									(279,994)	(86,344)	(85,844)
Consolidating Adjustments
Condensed Consolidating Statements of Operations
Net sales									(5,437,356)	(4,660,554)	(4,642,266)
Costs of goods sold									(5,326,933)	(4,573,242)	(4,571,416)
Gross profit									(110,423)	(87,312)	(70,850)
Selling, general and administrative									(18,149)	(16,435)	(16,005)
Operating income									(92,274)	(70,877)	(54,845)
Interest expense, net of capitalized interest									(4,930)	(4,771)	(5,289)
Other (income) expense, net									4,930	4,771	5,289
Income before income taxes									(92,274)	(70,877)	(54,845)
Income taxes (benefit)									(36,131)	(25,210)	(16,084)
Net income									(56,143)	(45,667)	(38,761)
Equity in net loss of subsidiaries									181,339	39,138	29,183
Net income attributable to Steel Dynamics, Inc.									$ 125,196	$ (6,529)	$ (9,578)