Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information
Net sales	$ 2,903,892	$ 3,223,547	$ 3,090,525	$ 2,603,875	$ 2,336,479	$ 2,443,382	$ 2,390,720	$ 2,368,216	$ 11,821,839	$ 9,538,797	$ 7,777,109
Gross profit	521,235	686,081	652,082	463,416	320,824	396,518	392,518	472,154	2,322,814	1,582,014	1,334,864
Operating income	365,553	531,572	501,887	323,397	195,947	271,015	265,353	334,566	1,722,409	1,066,881	727,966
Net income	269,852	397,906	362,572	225,475	303,171	151,012	152,947	198,666	1,255,805	805,796	360,006
Net income attributable to Steel Dynamics, Inc	$ 270,004	$ 398,375	$ 362,449	$ 227,551	$ 304,733	$ 153,258	$ 153,933	$ 200,817	$ 1,258,379	$ 812,741	$ 382,115
Earnings per share:
Basic (in dollars per share)	$ 1.18	$ 1.70	$ 1.54	$ 0.96	$ 1.28	$ 0.64	$ 0.64	$ 0.83	$ 5.38	$ 3.38	$ 1.57
Diluted (in dollars per share)	$ 1.17	$ 1.69	$ 1.53	$ 0.96	$ 1.28	$ 0.64	$ 0.63	$ 0.82	$ 5.35	$ 3.36	$ 1.56
Long Term Debt
Income Tax Expense (Benefit) Due to Tax Reform										$ (180,600)