Condensed Consolidating Information - Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Consolidating Balance Sheets
Ownership interest in subsidiaries (as a percent)	100.00%
Cash and equivalents	$ 727,032	$ 361,363	$ 395,156	$ 375,917
Accounts receivable, net	613,605	902,825
Inventories	1,149,390	1,618,419
Other current assets	47,914	55,655
Total current assets	2,537,941	2,938,262
Property, plant and equipment, net	2,951,210	3,123,906
Intangible assets, net	278,960	370,669
Goodwill	397,470	745,158
Other assets, including investments in subs	36,501	55,164
Total assets	6,202,082	7,233,159	5,888,534
Accounts payable	283,355	511,056
Accrued expenses	235,255	293,066
Current maturities of long-term debt	16,680	46,460
Total current liabilities	535,290	850,582
Long-term debt	2,577,976	2,935,389
Other liabilities	417,365	525,321
Redeemable noncontrolling interests	126,340	126,340
Common stock	638	635
Treasury stock	(396,455)	(398,898)
Additional paid-in capital	1,110,253	1,083,435
Retained earnings (deficit)	1,965,291	2,227,843
Total Steel Dynamics, Inc. equity	2,679,727	2,913,015
Noncontrolling interests	(134,616)	(117,488)
Total equity	2,545,111	2,795,527	2,495,855	2,377,842
Total liabilities and equity	6,202,082	7,233,159
Reportable legal entity | Parent
Condensed Consolidating Balance Sheets
Cash and equivalents	636,877	265,313	320,866	322,707
Accounts receivable, net	200,094	321,493
Inventories	539,963	662,970
Other current assets	21,654	59,243
Total current assets	1,398,588	1,309,019
Property, plant and equipment, net	958,212	1,002,407
Other assets, including investments in subs	2,941,710	3,858,706
Total assets	5,298,510	6,170,132
Accounts payable	100,751	151,517
Accrued expenses	141,552	191,433
Current maturities of long-term debt	13,122	13,073
Total current liabilities	255,425	356,023
Long-term debt	2,546,606	2,900,375
Other liabilities	(183,248)	719
Common stock	638	635
Treasury stock	(396,455)	(398,898)
Additional paid-in capital	1,110,253	1,083,435
Retained earnings (deficit)	1,965,291	2,227,843
Total Steel Dynamics, Inc. equity	2,679,727	2,913,015
Total equity	2,679,727	2,913,015
Total liabilities and equity	5,298,510	6,170,132
Reportable legal entity | Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	81,976	81,690	61,148	41,675
Accounts receivable, net	1,056,285	1,176,849
Inventories	573,924	862,796
Other current assets	25,415	8,416
Total current assets	1,737,600	2,129,751
Property, plant and equipment, net	1,703,932	1,826,208
Intangible assets, net	278,960	370,669
Goodwill	397,470	745,158
Other assets, including investments in subs	10,040	24,810
Total assets	4,128,002	5,096,596
Accounts payable	183,344	371,037
Accrued expenses	185,873	166,101
Current maturities of long-term debt	700	777
Total current liabilities	369,917	537,915
Long-term debt	361	624
Other liabilities	1,342,541	1,807,989
Common stock	1,727,859	1,727,859
Additional paid-in capital	117,737	117,737
Retained earnings (deficit)	569,587	904,472
Total Steel Dynamics, Inc. equity	2,415,183	2,750,068
Total equity	2,415,183	2,750,068
Total liabilities and equity	4,128,002	5,096,596
Reportable legal entity | Combined Non-Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	8,179	14,360	$ 13,142	$ 11,535
Accounts receivable, net	29,775	44,696
Inventories	35,004	94,916
Other current assets	1,676	6,465
Total current assets	74,634	160,437
Property, plant and equipment, net	291,077	297,505
Other assets, including investments in subs	6,137	6,255
Total assets	371,848	464,197
Accounts payable	68,948	98,886
Accrued expenses	4,779	11,695
Current maturities of long-term debt	24,975	73,767
Total current liabilities	98,702	184,348
Long-term debt	177,897	158,333
Other liabilities	63,020	28,559
Redeemable noncontrolling interests	126,340	126,340
Common stock	18,120	18,121
Additional paid-in capital	646,787	635,156
Retained earnings (deficit)	(624,402)	(569,172)
Total Steel Dynamics, Inc. equity	40,505	84,105
Noncontrolling interests	(134,616)	(117,488)
Total equity	(94,111)	(33,383)
Total liabilities and equity	371,848	464,197
Consolidating Adjustments
Condensed Consolidating Balance Sheets
Accounts receivable, net	(672,549)	(640,213)
Inventories	499	(2,263)
Other current assets	(831)	(18,469)
Total current assets	(672,881)	(660,945)
Property, plant and equipment, net	(2,011)	(2,214)
Other assets, including investments in subs	(2,921,386)	(3,834,607)
Total assets	(3,596,278)	(4,497,766)
Accounts payable	(69,688)	(110,384)
Accrued expenses	(96,949)	(76,163)
Current maturities of long-term debt	(22,117)	(41,157)
Total current liabilities	(188,754)	(227,704)
Long-term debt	(146,888)	(123,943)
Other liabilities	(804,948)	(1,311,946)
Common stock	(1,745,979)	(1,745,980)
Additional paid-in capital	(764,524)	(752,893)
Retained earnings (deficit)	54,815	(335,300)
Total Steel Dynamics, Inc. equity	(2,455,688)	(2,834,173)
Total equity	(2,455,688)	(2,834,173)
Total liabilities and equity	$ (3,596,278)	$ (4,497,766)