Quarterly Financial Information (unaudited, in thousands, except per share data) (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
	Mar. 16, 2015	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net sales		$ 1,591,046	$ 1,950,923	$ 2,005,007	$ 2,047,435	$ 2,517,093	$ 2,339,016	$ 2,069,761	$ 1,830,082	$ 7,594,411	$ 8,755,952	$ 7,372,924
Gross profit		144,207	228,726	171,743	187,042	291,624	288,512	222,771	163,304	731,718	966,211	719,144
Operating income (loss)		(381,143)	131,029	77,559	99,771	(81,160)	188,643	131,905	80,932	(72,784)	320,320	386,525
Net income (loss)		(256,316)	58,867	25,325	26,954	(96,046)	87,657	66,341	33,698	(145,170)	91,650	163,516
Net income (loss) attributable to Steel Dynamics, Inc.		$ (253,239)	$ 60,617	$ 31,550	$ 30,761	$ (45,031)	$ 91,173	$ 72,303	$ 38,579	(130,311)	157,024	189,314
Asset impairment charges										$ 428,500	$ 260,000	$ 308
Earnings (loss) per share:
Basic (in dollars per share)		$ (1.04)	$ 0.25	$ 0.13	$ 0.13	$ (0.19)	$ 0.38	$ 0.32	$ 0.17	$ (0.54)	$ 0.68	$ 0.86
Diluted (in dollars per share)		$ (1.04)	$ 0.25	$ 0.13	$ 0.13	$ (0.19)	$ 0.38	$ 0.31	$ 0.17	$ (0.54)	$ 0.67	$ 0.83
Long Term Debt
Call and repayment of debt										$ 612,534	$ 635,578	$ 517,978
Severstal Columbus LLC
Other non-operating expenses							$ 25,200				25,200
Operating Segment
Net sales										7,594,411	8,755,952	7,372,924
Operating Segment | Metals Recycling Operations
Net sales										2,337,716	3,539,206	3,274,866
Operating income (loss)										(448,137)	$ 26,014	$ 41,949
Net income (loss) attributable to Steel Dynamics, Inc.		$ 268,700
Asset impairment charges		$ 428,500								$ 428,500
Senior Notes
Long Term Debt
Call premiums and write off of deferred financing costs					$ 16,700
Call and repayment of debt					$ 350,000
7 5/8% senior notes, due 2020
Long Term Debt
Call premiums and write off of deferred financing costs	$ 16,700
Call and repayment of debt	$ 350,000
Stated interest rate (as a percent)		7.625%			7.625%	7.625%				7.625%	7.625%
Minnesota Ironmaking Operations
Operating income (loss)						$ (260,000)
Net income (loss)						(179,100)
Net income (loss) attributable to Steel Dynamics, Inc.						$ (132,600)
Cost of goods sold | Minnesota Ironmaking Operations
Inventory lower-of-cost or market charge				$ 21,000
Noncontrolling Interests | Cost of goods sold | Minnesota Ironmaking Operations
Inventory lower-of-cost or market charge				$ 3,600