Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information
Net sales	$ 2,601,245	$ 2,330,832	$ 2,094,305	$ 2,575,100	$ 2,350,196	$ 2,526,845	$ 2,770,515	$ 2,817,435	$ 9,601,482	$ 10,464,991	$ 11,821,839
Gross profit	442,253	292,815	284,431	415,229	316,409	359,839	421,166	433,570	1,434,728	1,530,984	2,322,814
Operating income	258,750	155,856	158,850	273,686	181,961	228,045	285,032	291,842	847,142	986,880	1,722,409
Net income	196,950	104,277	78,765	190,836	123,719	152,608	196,746	204,827	570,828	677,900	1,255,805
Net income attributable to Steel Dynamics, Inc	$ 187,843	$ 100,143	$ 75,496	$ 187,340	$ 121,425	$ 151,048	$ 194,302	$ 204,328	$ 550,822	$ 671,103	$ 1,258,379
Earnings per share:
Basic (in dollars per share)	$ 0.89	$ 0.48	$ 0.36	$ 0.88	$ 0.56	$ 0.69	$ 0.88	$ 0.91	$ 2.61	$ 3.06	$ 5.38
Diluted (in dollars per share)	$ 0.89	$ 0.47	$ 0.36	$ 0.88	$ 0.56	$ 0.69	$ 0.87	$ 0.91	$ 2.59	$ 3.04	$ 5.35