Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information
Net sales	$ 2,350,196	$ 2,526,845	$ 2,770,515	$ 2,817,435	$ 2,903,892	$ 3,223,547	$ 3,090,525	$ 2,603,875		$ 10,464,991	$ 11,821,839	$ 9,538,797
Gross profit	316,409	359,839	421,166	433,570	521,235	686,081	652,082	463,416		1,530,984	2,322,814	1,582,014
Operating income	181,961	228,045	285,032	291,842	365,553	531,572	501,887	323,397		986,880	1,722,409	1,066,881
Net income	123,719	152,608	196,746	204,827	269,852	397,906	362,572	225,475		677,900	1,255,805	805,796
Net income attributable to Steel Dynamics, Inc	$ 121,425	$ 151,048	$ 194,302	$ 204,328	$ 270,004	$ 398,375	$ 362,449	$ 227,551		$ 671,103	$ 1,258,379	$ 812,741
Earnings per share:
Basic (in dollars per share)	$ 0.56	$ 0.69	$ 0.88	$ 0.91	$ 1.18	$ 1.70	$ 1.54	$ 0.96		$ 3.06	$ 5.38	$ 3.38
Diluted (in dollars per share)	$ 0.56	$ 0.69	$ 0.87	$ 0.91	$ 1.17	$ 1.69	$ 1.53	$ 0.96		$ 3.04	$ 5.35	$ 3.36
Long Term Debt
Income Tax Expense (Benefit) Due to Tax Reform									$ (180,600)