CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 1,255,805	$ 805,796	$ 360,006
Other comprehensive income - net unrealized gain on cash flow hedging derivatives, net of income tax expense of $94 for 2018	301
Comprehensive income	1,256,106	805,796	360,006
Comprehensive loss attributable to noncontrolling interests	2,574	6,945	22,109
Comprehensive income attributable to Steel Dynamics, Inc.	$ 1,258,680	$ 812,741	$ 382,115