CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net sales
Unrelated parties	$ 9,364,213	$ 7,607,180	$ 7,407,233
Related parties	174,584	169,929	187,178
Total net sales	9,538,797	7,777,109	7,594,411
Costs of goods sold	7,956,783	6,442,245	6,862,693
Gross profit	1,582,014	1,334,864	731,718
Selling, general and administrative expenses	394,631	374,009	328,758
Profit sharing	91,309	71,285	23,064
Amortization of intangible assets	29,193	28,765	24,180
Asset impairment charges		132,839	428,500
Operating income	1,066,881	727,966	(72,784)
Interest expense, net of capitalized interest	134,399	146,037	153,950
Other expense (income), net	(2,753)	17,796	15,383
Income before income taxes	935,235	564,133	(242,117)
Income tax expense	129,439	204,127	(96,947)
Net income	805,796	360,006	(145,170)
Net loss attributable to noncontrolling interests	6,945	22,109	14,859
Net income attributable to Steel Dynamics, Inc.	$ 812,741	$ 382,115	$ (130,311)
Basic earnings per share attributable to Steel Dynamics, Inc. stockholders	$ 3.38	$ 1.57	$ (0.54)
Weighted average common shares outstanding - basic (in shares)	240,132	243,576	242,017
Diluted earnings per share attributable to Steel Dynamics, Inc. stockholders, including the effect of assumed conversions when dilutive (in dollars per share)	$ 3.36	$ 1.56	$ (0.54)
Weighted average common shares and share equivalents outstanding (in shares)	241,781	245,298	242,017
Dividends declared per share (in dollars per share)	$ 0.62	$ 0.56	$ 0.55