Quarterly Financial Information (unaudited, in thousands, except per share data) (Details) (USD $) In Thousands, except Per Share data, 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
Quarterly Financial Information (unaudited, in thousands, except per share data)
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
Gross profit	160,135	199,243	276,386	295,754	133,668	139,532	191,984	210,482	931,518	675,666	399,076
Operating income	82,490	108,785	188,219	205,326	46,821	69,000	116,635	132,297	584,820	364,753	119,531
Net income	25,779	39,857	95,826	104,230	4,058	15,355	46,797	63,389	265,692	129,599	(11,019)
Net income 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)
Earnings per share:
Basic	$ 0.14	$ 0.20	$ 0.45	$ 0.49	$ 0.04	$ 0.09	$ 0.23	$ 0.30	$ 1.27	$ 0.65	$ (0.04)
Diluted	$ 0.14	$ 0.19	$ 0.43	$ 0.46	$ 0.04	$ 0.09	$ 0.22	$ 0.29	$ 1.22	$ 0.64	$ (0.04)