Description of the Business and Summary of Significant Accounting Policies - Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jun. 15, 2014	Jun. 12, 2014
Antidilutive securities excluded from computation of earnings per share amount
Antidilutive securities excluded from computation of earnings per share amount (in shares)									1,500	0	0
Net Income (Loss) (Numerator)
Basic earnings per share - net income (in dollars)	$ (253,239)	$ 60,617	$ 31,550	$ 30,761	$ (45,031)	$ 91,173	$ 72,303	$ 38,579	$ (130,311)	$ 157,024	$ 189,314
5.125% convertible senior notes, net of tax (in dollars)										4,327	9,432
Diluted earnings per share - net income (in dollars)									$ (130,311)	$ 161,351	$ 198,746
Shares (Denominator)
Weighted average common shares outstanding - basic (in shares)									242,017	232,547	220,916
Dilutive stock options, deferred stock units, and restricted stock units (in shares)										1,828	1,392
5.125% convertible senior notes, net of tax (in shares)										7,703	16,688
Weighted average common shares outstanding - diluted (in shares)									242,017	242,078	238,996
Per Share Amount
Basic earnings (loss) per share attributable to Steel Dynamics, Inc. stockholders (in dollars per share)	$ (1.04)	$ 0.25	$ 0.13	$ 0.13	$ (0.19)	$ 0.38	$ 0.32	$ 0.17	$ (0.54)	$ 0.68	$ 0.86
Diluted earnings (loss) per share attributable to Steel Dynamics, Inc. stockholders, including the effect of assumed conversions when dilutive (in dollars per share)	$ (1.04)	$ 0.25	$ 0.13	$ 0.13	$ (0.19)	$ 0.38	$ 0.31	$ 0.17	$ (0.54)	$ 0.67	$ 0.83
5.125% convertible senior notes
Per Share Amount
Stated interest rate (as a percent)	5.125%				5.125%				5.125%	5.125%	5.125%	5.125%	5.125%