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Description of the Business and Summary of Significant Accounting Policies (Details 5) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 15, 2014	Jun. 12, 2014
Net Income (Numerator)
Basic earnings - net income (loss) (in dollars)	$ (45,031)	$ 91,173	$ 72,303	$ 38,579	$ 54,655	$ 57,486	$ 28,958	$ 48,215	$ 157,024	$ 189,314	$ 163,551
5.125% convertible senior notes, net of tax (in dollars)									4,327	9,432	9,432
Diluted earnings - net income (loss) (in dollars)									$ 161,351	$ 198,746	$ 172,983
Weighted average common shares outstanding (in shares)									232,547,000	220,916,000	219,159,000
Dilutive stock options, deferred stock units, and restricted stock units (in shares)									1,828,000	1,392,000	916,000
5.125% convertible senior notes, net of tax (in shares)									7,703,000	16,688,000	16,549,000
Weighted average common shares outstanding - diluted (in shares)									242,078,000	238,996,000	236,624,000
Per Share Amount
Basic earnings per share attributable to Steel Dynamics, Inc. stockholders (in dollars per share)	$ (0.19)us-gaap_EarningsPerShareBasic	$ 0.38us-gaap_EarningsPerShareBasic	$ 0.32us-gaap_EarningsPerShareBasic	$ 0.17us-gaap_EarningsPerShareBasic	$ 0.25us-gaap_EarningsPerShareBasic	$ 0.26us-gaap_EarningsPerShareBasic	$ 0.13us-gaap_EarningsPerShareBasic	$ 0.22us-gaap_EarningsPerShareBasic	$ 0.68us-gaap_EarningsPerShareBasic	$ 0.86us-gaap_EarningsPerShareBasic	$ 0.75us-gaap_EarningsPerShareBasic
Diluted earnings per share attributable to Steel Dynamics, Inc. stockholders, including the effect of assumed conversions when dilutive (in dollars per share)	$ (0.19)us-gaap_EarningsPerShareDiluted	$ 0.38us-gaap_EarningsPerShareDiluted	$ 0.31us-gaap_EarningsPerShareDiluted	$ 0.17us-gaap_EarningsPerShareDiluted	$ 0.24us-gaap_EarningsPerShareDiluted	$ 0.25us-gaap_EarningsPerShareDiluted	$ 0.13us-gaap_EarningsPerShareDiluted	$ 0.21us-gaap_EarningsPerShareDiluted	$ 0.67us-gaap_EarningsPerShareDiluted	$ 0.83us-gaap_EarningsPerShareDiluted	$ 0.73us-gaap_EarningsPerShareDiluted
5.125% convertible senior notes
Antidilutive securities excluded from computation of earnings per share amount
Stated interest rate (as a percent)	5.125%				5.125%				5.125%	5.125%	5.125%	5.125%	5.125%
Options
Antidilutive securities excluded from computation of earnings per share amount
Antidilutive securities excluded from computation of earnings per share amount (in shares)									0	0