Condensed Consolidating Information (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Condensed Consolidating Financial Statements
Net sales	$ 1,858,345	$ 2,043,455	$ 2,079,731	$ 2,015,969	$ 1,528,134	$ 1,584,164	$ 1,632,799	$ 1,555,790	$ 7,997,500	$ 6,300,887	$ 3,958,806
Costs of goods sold									7,065,982	5,625,221	3,559,730
Gross profit (loss)	160,135	199,243	276,386	295,754	133,668	139,532	191,984	210,482	931,518	675,666	399,076
Selling, general and administrative									346,698	310,913	279,545
Operating income (loss)	82,490	108,785	188,219	205,326	46,821	69,000	116,635	132,297	584,820	364,753	119,531
Interest expense, net of capitalized interest									176,977	170,229	141,360
Other (income) expense, net									(16,476)	(18,935)	(3,592)
Income (loss) before income taxes and equity in net income of subsidiaries									424,319	213,459	(18,237)
Income taxes (benefit)									158,627	83,860	(7,218)
Net loss	25,779	39,857	95,826	104,230	4,058	15,355	46,797	63,389	265,692	129,599	(11,019)
Net loss attributable to noncontrolling interests									12,428	11,110	2,835
Net income (loss) attributable to Steel Dynamics, Inc.	30,203	43,304	98,710	105,903	7,792	18,741	49,207	64,969	278,120	140,709	(8,184)
Parent
Condensed Consolidating Financial Statements
Net sales									3,701,928	2,866,541	1,790,451
Costs of goods sold									3,099,941	2,467,780	1,628,271
Gross profit (loss)									601,987	398,761	162,180
Selling, general and administrative									135,509	98,410	68,633
Operating income (loss)									466,478	300,351	93,547
Interest expense, net of capitalized interest									104,008	99,639	83,297
Other (income) expense, net									(12,663)	(14,629)	49,255
Income (loss) before income taxes and equity in net income of subsidiaries									375,133	215,341	(39,005)
Income taxes (benefit)									111,532	84,718	(15,438)
Net loss									263,601	130,623	(23,567)
Equity in net income of subsidiaries									14,519	10,086	15,383
Net income (loss) attributable to Steel Dynamics, Inc.									278,120	140,709	(8,184)
Guarantors
Condensed Consolidating Financial Statements
Net sales									9,062,305	7,067,399	4,334,533
Costs of goods sold									8,604,620	6,701,621	4,055,388
Gross profit (loss)									457,685	365,778	279,145
Selling, general and administrative									217,717	215,723	208,847
Operating income (loss)									239,968	150,055	70,298
Interest expense, net of capitalized interest									71,487	67,659	58,049
Other (income) expense, net									(8,012)	(15,123)	(55,700)
Income (loss) before income taxes and equity in net income of subsidiaries									176,493	97,519	67,949
Income taxes (benefit)									66,734	36,325	26,894
Net loss									109,759	61,194	41,055
Net income (loss) attributable to Steel Dynamics, Inc.									109,759	61,194	41,055
Combined Non-Guarantors
Condensed Consolidating Financial Statements
Net sales									190,739	118,141	54,053
Costs of goods sold									251,528	164,400	52,996
Gross profit (loss)									(60,789)	(46,259)	1,057
Selling, general and administrative									9,184	8,724	15,057
Operating income (loss)									(69,973)	(54,983)	(14,000)
Interest expense, net of capitalized interest									8,784	13,168	735
Other (income) expense, net									(3,209)	(296)	9
Income (loss) before income taxes and equity in net income of subsidiaries									(75,548)	(67,855)	(14,744)
Income taxes (benefit)									(151)	(26,477)	(5,835)
Net loss									(75,397)	(41,378)	(8,909)
Net loss attributable to noncontrolling interests									12,428	11,110	2,835
Net income (loss) attributable to Steel Dynamics, Inc.									(62,969)	(30,268)	(6,074)
Consolidating Adjustments
Condensed Consolidating Financial Statements
Net sales									(4,957,472)	(3,751,194)	(2,220,231)
Costs of goods sold									(4,890,107)	(3,708,580)	(2,176,925)
Gross profit (loss)									(67,365)	(42,614)	(43,306)
Selling, general and administrative									(15,712)	(11,944)	(12,992)
Operating income (loss)									(51,653)	(30,670)	(30,314)
Interest expense, net of capitalized interest									(7,302)	(10,237)	(721)
Other (income) expense, net									7,408	11,113	2,844
Income (loss) before income taxes and equity in net income of subsidiaries									(51,759)	(31,546)	(32,437)
Income taxes (benefit)									(19,488)	(10,706)	(12,839)
Net loss									(32,271)	(20,840)	(19,598)
Equity in net income of subsidiaries									(14,519)	(10,086)	(15,383)
Net income (loss) attributable to Steel Dynamics, Inc.									$ (46,790)	$ (30,926)	$ (34,981)