Amount for expansion to add products such as painted and Galvalume capacity in construction Phase.

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- Definition

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 coating lines.

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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.

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Represents the number of plants.

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Number of years subsequent to achievement of certain performance measures.

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No definition available.

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Percentage of ownership interest in facility.

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No definition available.

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- Definition

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

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

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

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- Definition

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

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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Description of the Business and Summary of Significant Accounting Policies - Segments (Details) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($) $ / shares	Sep. 30, 2015 USD ($) $ / shares	Jun. 30, 2015 USD ($) $ / shares	Mar. 31, 2015 USD ($) $ / shares	Dec. 31, 2014 USD ($) $ / shares	Sep. 30, 2014 USD ($) $ / shares	Jun. 30, 2014 USD ($) $ / shares	Mar. 31, 2014 USD ($) $ / shares	Dec. 31, 2015 USD ($) segment location employee Plant facility item agreement $ / shares	Dec. 31, 2014 USD ($) $ / shares	Dec. 31, 2013 USD ($) $ / shares
Segment Reporting Information
Number of reporting segments \| segment									3
Workforce represented by collective bargaining agreements (as a percent)									9.00%
Number of collective bargaining agreements that expire \| agreement									1
Number of employees under a collective bargaining agreement that expire \| employee									117
Number of locations subject to collective bargaining agreements that expire \| location									1
Net income (loss)	$ (256,316)	$ 58,867	$ 25,325	$ 26,954	$ (96,046)	$ 87,657	$ 66,341	$ 33,698	$ (145,170)	$ 91,650	$ 163,516
Diluted earnings (loss) per share attributable to Steel Dynamics, Inc. stockholders, including the effect of assumed conversions when dilutive (in dollars per share) \| $ / shares	$ (1.04)	$ 0.25	$ 0.13	$ 0.13	$ (0.19)	$ 0.38	$ 0.31	$ 0.17	$ (0.54)	$ 0.67	$ 0.83
Other
Redeemable noncontrolling interests	$ 126,340				$ 126,340				$ 126,340	$ 126,340
Minnesota Ironmaking Operations
Segment Reporting Information
Net income (loss)					(179,100)
Subsidiaries \| Mesabi Nugget
Other
Percentage of ownership interest in facility									82.00%
Number of years subsequent to achievement of performance measures									3 years
Subsidiaries \| Mesabi Nugget \| Redeemable Noncontrolling Interest
Other
Redeemable noncontrolling interests	111,200				111,200				$ 111,200	111,200
Subsidiaries \| Mining Resources
Other
Percentage of ownership interest in facility									81.00%
Subsidiaries \| Mining Resources \| Redeemable Noncontrolling Interest
Other
Redeemable noncontrolling interests	15,100				$ 15,100				$ 15,100	$ 15,100
Steel Operations
Segment Reporting Information
Number of downstream coating lines \| facility									10
Percentage of external net sales									69.00%	63.00%	61.00%
Steel Operations \| Columbus Flat Roll Division
Segment Reporting Information
Amount for expansion project	$ 100,000								$ 100,000
Steel Operations \| The Techs
Segment Reporting Information
Number of galvanizing lines \| item									3
Metals Recycling Operations
Segment Reporting Information
Percentage of external net sales									19.00%	25.00%	31.00%
Steel Fabrication Operations
Segment Reporting Information
Percentage of external net sales									9.00%	7.00%	6.00%
Number of plants \| Plant									8