Number of agreements under collective bargaining arrangements that are to expire within the next twelve months.

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- Definition

Represents the number of downstream finishing facilities.

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No definition available.

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- Definition

Represents number of galvanizing lines to be used in galvanizing of specific types of flat roll steels in non-automotive applications.

+ References

No definition available.

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- Definition

Represents the number of ironmaking initiatives.

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No definition available.

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- Definition

Represents the number of plants.

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No definition available.

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- Definition

Number of years subsequent to achievement of certain performance measures.

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No definition available.

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- Definition

Percentage of ownership interest in facility.

+ References

No definition available.

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- Definition

The percentage of employees covered by a collective bargaining agreement as of the balance sheet date.

+ References

No definition available.

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- Definition

Percentage of net revenue of the entity's external sales accounted for by the reportable unit.

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No definition available.

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- Definition

The number of employees covered by a collective bargaining agreement that expires within the next twelve months.

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No definition available.

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- Definition

The number of locations under collective bargaining arrangements that expire within the next twelve months.

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No definition available.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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No definition available.

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- Definition

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

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-Name Accounting Standards Codification

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As of the reporting date, the aggregate carrying amount of all noncontrolling interests which are redeemable by the (parent) entity (1) at a fixed or determinable price on a fixed or determinable date, (2) at the option of the holder of the noncontrolling interest, or (3) upon occurrence of an event that is not solely within the control of the (parent) entity. This item includes noncontrolling interest holder's ownership (or holders' ownership) regardless of the type of equity interest (common, preferred, other) including all potential organizational (legal) forms of the investee entity.

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Description of the Business and Summary of Significant Accounting Policies (Details) (USD $) In Thousands, except Per 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 segment agreement location employee	Dec. 31, 2013	Dec. 31, 2012
Description of the Business and Summary of Significant Accounting Policies
Number of reporting segments									3us-gaap_NumberOfReportableSegments
Workforce represented by collective bargaining agreements (as a percent)									9.00%
Number of collective bargaining agreements that expire									1
Number of employees under a collective bargaining agreement that expire									35
Number of locations subject to collective bargaining agreements that expire									1
Segment Reporting Information
Net income	$ (96,046)us-gaap_ProfitLoss	$ 87,657us-gaap_ProfitLoss	$ 66,341us-gaap_ProfitLoss	$ 33,698us-gaap_ProfitLoss	$ 47,901us-gaap_ProfitLoss	$ 51,090us-gaap_ProfitLoss	$ 23,273us-gaap_ProfitLoss	$ 41,252us-gaap_ProfitLoss	$ 91,650us-gaap_ProfitLoss	$ 163,516us-gaap_ProfitLoss	$ 142,281us-gaap_ProfitLoss
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
Redeemable noncontrolling interests	126,340				116,514				126,340	116,514
Minnesota Ironmaking Operations
Segment Reporting Information
Net income	(179,100)
Subsidiaries \| Mesabi Nugget
Segment Reporting Information
Number of years subsequent to achievement of performance measures									3 years
Steel Operations
Segment Reporting Information
Number of downstream finishing facilities									8
Percentage of external net sales									63.00%	61.00%	62.00%
Steel Operations \| The Techs Operations
Segment Reporting Information
Number of galvanizing lines									3
Metals Recycling/Ferrous Resources
Segment Reporting Information
Percentage of external net sales									28.00%	32.00%	32.00%
Number of ironmaking initiatives									2
Metals Recycling/Ferrous Resources \| Mesabi Nugget
Segment Reporting Information
Percentage of ownership interest in facility									82.00%
Redeemable noncontrolling interests	111,200				101,400				111,200	101,400
Metals Recycling/Ferrous Resources \| Mining Resources
Segment Reporting Information
Percentage of ownership interest in facility									81.00%
Redeemable noncontrolling interests	$ 15,100				$ 15,100				$ 15,100	$ 15,100
Steel Fabrication Operations
Segment Reporting Information
Percentage of external net sales									7.00%	6.00%	5.00%
Number of plants									6