Quarterly Financial Information (unaudited, in thousands, except per share data) (Details) (USD $)	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
Quarterly Financial Information (unaudited, in thousands, except per share data)
Net sales	$ 1,705,001,000	$ 1,693,390,000	$ 1,909,803,000	$ 1,982,040,000	$ 1,858,345,000	$ 2,043,455,000	$ 2,079,731,000	$ 2,015,969,000	$ 7,290,234,000	$ 7,997,500,000	$ 6,300,887,000
Gross profit	180,097,000	156,401,000	182,136,000	201,264,000	160,135,000	199,243,000	276,386,000	295,754,000	719,898,000	931,518,000	675,666,000
Operating income	94,929,000	72,721,000	103,699,000	119,816,000	82,490,000	108,785,000	188,219,000	205,326,000	391,165,000	584,820,000	364,753,000
Net income	55,094,000	6,105,000	39,305,000	41,777,000	25,779,000	39,857,000	95,826,000	104,230,000	142,281,000	265,692,000	129,599,000
Net income attributable to Steel Dynamics, Inc.	60,571,000	12,833,000	44,472,000	45,675,000	30,203,000	43,304,000	98,710,000	105,903,000	163,551,000	278,120,000	140,709,000
Earnings per share:
Basic (in dollars per share)	$ 0.28	$ 0.06	$ 0.20	$ 0.21	$ 0.14	$ 0.20	$ 0.45	$ 0.49	$ 0.75	$ 1.27	$ 0.65
Diluted (in dollars per share)	$ 0.27	$ 0.06	$ 0.20	$ 0.20	$ 0.14	$ 0.19	$ 0.43	$ 0.46	$ 0.73	$ 1.22	$ 0.64
Other non-operating expenses		$ 26,400,000		$ 13,900,000					$ 40,300,000