Quarterly Financial Information (unaudited, in thousands, except per share data) (Details) (USD $)	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
Revenue, Net	$ 2,517,093,000	$ 2,339,016,000	$ 2,069,761,000	$ 1,830,082,000	$ 1,864,150,000	$ 1,911,738,000	$ 1,801,340,000	$ 1,795,696,000	$ 8,755,952,000	$ 7,372,924,000	$ 7,290,234,000
Gross Profit	291,624,000	288,512,000	222,771,000	163,304,000	197,996,000	197,192,000	147,692,000	176,264,000	966,211,000	719,144,000	719,898,000
Operating income (loss)	(81,160,000)	188,643,000	131,905,000	80,932,000	107,822,000	113,273,000	69,198,000	96,232,000	320,320,000	386,525,000	391,165,000
Net income (loss)	(96,046,000)	87,657,000	66,341,000	33,698,000	47,901,000	51,090,000	23,273,000	41,252,000	91,650,000	163,516,000	142,281,000
Net income (loss) attributable to Steel Dynamics, Inc.	(45,031,000)	91,173,000	72,303,000	38,579,000	54,655,000	57,486,000	28,958,000	48,215,000	157,024,000	189,314,000	163,551,000
Basic (in dollars per share)	$ (0.19)	$ 0.38	$ 0.32	$ 0.17	$ 0.25	$ 0.26	$ 0.13	$ 0.22	$ 0.68	$ 0.86	$ 0.75
Diluted (in dollars per share)	$ (0.19)	$ 0.38	$ 0.31	$ 0.17	$ 0.24	$ 0.25	$ 0.13	$ 0.21	$ 0.67	$ 0.83	$ 0.73
Other non-operating expenses		25,000,000
Minnesota Ironmaking Operations
Operating income (loss)	(260,000,000)
Net income (loss)	(179,100,000)
Net income (loss) attributable to Steel Dynamics, Inc.	$ (132,600,000)