Condensed Consolidating Information (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Condensed Consolidating Financial Statements
Net sales	$ 1,705,001	$ 1,693,390	$ 1,909,803	$ 1,982,040	$ 1,858,345	$ 2,043,455	$ 2,079,731	$ 2,015,969	$ 7,290,234	$ 7,997,500	$ 6,300,887
Costs of goods sold									6,570,336	7,065,982	5,625,221
Gross profit	180,097	156,401	182,136	201,264	160,135	199,243	276,386	295,754	719,898	931,518	675,666
Selling, general and administrative									328,733	346,698	310,913
Operating income	94,929	72,721	103,699	119,816	82,490	108,785	188,219	205,326	391,165	584,820	364,753
Interest expense, net of capitalized interest									158,585	176,977	170,229
Other (income) expense, net									28,514	(16,476)	(18,935)
Income before income taxes									204,066	424,319	213,459
Income taxes (benefit)									61,785	158,627	83,860
Net income	55,094	6,105	39,305	41,777	25,779	39,857	95,826	104,230	142,281	265,692	129,599
Net loss attributable to noncontrolling interests									21,270	12,428	11,110
Net income attributable to Steel Dynamics, Inc.	60,571	12,833	44,472	45,675	30,203	43,304	98,710	105,903	163,551	278,120	140,709
Parent
Condensed Consolidating Financial Statements
Net sales									3,431,978	3,701,928	2,866,541
Costs of goods sold									2,977,756	3,099,941	2,467,780
Gross profit									454,222	601,987	398,761
Selling, general and administrative									102,142	135,509	98,410
Operating income									352,080	466,478	300,351
Interest expense, net of capitalized interest									104,058	104,008	99,639
Other (income) expense, net									28,677	(12,663)	(14,629)
Income before income taxes									219,345	375,133	215,341
Income taxes (benefit)									26,611	111,532	84,718
Net income									192,734	263,601	130,623
Equity in net loss of subsidiaries									(29,183)	14,519	10,086
Net income attributable to Steel Dynamics, Inc.									163,551	278,120	140,709
Guarantors
Condensed Consolidating Financial Statements
Net sales									8,322,995	9,062,305	7,067,399
Costs of goods sold									7,906,392	8,604,620	6,701,621
Gross profit									416,603	457,685	365,778
Selling, general and administrative									218,515	217,717	215,723
Operating income									198,088	239,968	150,055
Interest expense, net of capitalized interest									53,186	71,487	67,659
Other (income) expense, net									(1,531)	(8,012)	(15,123)
Income before income taxes									146,433	176,493	97,519
Income taxes (benefit)									51,011	66,734	36,325
Net income									95,422	109,759	61,194
Net income attributable to Steel Dynamics, Inc.									95,422	109,759	61,194
Combined Non-Guarantors
Condensed Consolidating Financial Statements
Net sales									177,527	190,739	118,141
Costs of goods sold									257,604	251,528	164,400
Gross profit									(80,077)	(60,789)	(46,259)
Selling, general and administrative									24,081	9,184	8,724
Operating income									(104,158)	(69,973)	(54,983)
Interest expense, net of capitalized interest									6,630	8,784	13,168
Other (income) expense, net									(3,921)	(3,209)	(296)
Income before income taxes									(106,867)	(75,548)	(67,855)
Income taxes (benefit)									247	(151)	(26,477)
Net income									(107,114)	(75,397)	(41,378)
Net loss attributable to noncontrolling interests									21,270	12,428	11,110
Net income attributable to Steel Dynamics, Inc.									(85,844)	(62,969)	(30,268)
Consolidating Adjustments
Condensed Consolidating Financial Statements
Net sales									(4,642,266)	(4,957,472)	(3,751,194)
Costs of goods sold									(4,571,416)	(4,890,107)	(3,708,580)
Gross profit									(70,850)	(67,365)	(42,614)
Selling, general and administrative									(16,005)	(15,712)	(11,944)
Operating income									(54,845)	(51,653)	(30,670)
Interest expense, net of capitalized interest									(5,289)	(7,302)	(10,237)
Other (income) expense, net									5,289	7,408	11,113
Income before income taxes									(54,845)	(51,759)	(31,546)
Income taxes (benefit)									(16,084)	(19,488)	(10,706)
Net income									(38,761)	(32,271)	(20,840)
Equity in net loss of subsidiaries									29,183	(14,519)	(10,086)
Net income attributable to Steel Dynamics, Inc.									$ (9,578)	$ (46,790)	$ (30,926)