Condensed Consolidating Information (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Condensed Consolidating Balance Sheets
Ownership interest in subsidiaries (as a percent)	100.00%
Cash and equivalents	$ 361,363	$ 395,156	$ 375,917	$ 390,761
Accounts receivable, net	902,825	720,600
Inventories	1,618,419	1,314,747
Other current assets	91,158	43,131
Total current assets	2,973,765	2,473,634
Property, plant and equipment, net	3,123,906	2,226,134
Intangible assets, net	370,669	386,159
Goodwill	745,158	731,996
Other assets, including investments in subs	97,529	115,083
Total assets	7,311,027	5,933,006	5,815,416
Accounts payable	511,056	414,932
Accrued expenses	293,066	218,702
Current maturities of long-term debt	46,460	341,544
Total current liabilities	850,582	975,178
Long-term debt	2,977,706	1,766,045
Other liabilities	560,872	579,414
Redeemable noncontrolling interests	126,340	116,514
Common stock	635	645
Treasury stock	(398,898)	(718,529)
Additional paid-in capital	1,083,435	1,085,694
Retained earnings (deficit)	2,227,843	2,179,513
Total Steel Dynamics, Inc. equity	2,913,015	2,547,323
Noncontrolling interests	(117,488)	(51,468)
Total equity	2,795,527	2,495,855	2,377,842	2,299,900
Total liabilities and equity	7,311,027	5,933,006
Reportable legal entity | Parent
Condensed Consolidating Balance Sheets
Cash and equivalents	265,313	320,866	322,707	301,073
Accounts receivable, net	321,493	309,691
Inventories	662,970	673,763
Other current assets	94,634	50,228
Total current assets	1,344,410	1,354,548
Property, plant and equipment, net	1,002,407	1,046,093
Other assets, including investments in subs	3,900,691	2,630,411
Total assets	6,247,508	5,031,052
Accounts payable	151,517	160,255
Accrued expenses	191,433	142,055
Current maturities of long-term debt	13,073	315,521
Total current liabilities	356,023	617,831
Long-term debt	2,942,360	1,725,433
Other liabilities	36,110	140,465
Common stock	635	645
Treasury stock	(398,898)	(718,529)
Additional paid-in capital	1,083,435	1,085,694
Retained earnings (deficit)	2,227,843	2,179,513
Total Steel Dynamics, Inc. equity	2,913,015	2,547,323
Total equity	2,913,015	2,547,323
Total liabilities and equity	6,247,508	5,031,052
Reportable legal entity | Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	81,690	61,148	41,675	58,699
Accounts receivable, net	1,176,849	874,707
Inventories	862,796	557,640
Other current assets	8,416	8,399
Total current assets	2,129,751	1,501,894
Property, plant and equipment, net	1,826,208	619,617
Intangible assets, net	370,669	386,159
Goodwill	745,158	731,996
Other assets, including investments in subs	24,810	21,789
Total assets	5,096,596	3,261,455
Accounts payable	371,037	258,406
Accrued expenses	166,101	115,182
Current maturities of long-term debt	777	300
Total current liabilities	537,915	373,888
Long-term debt	624
Other liabilities	1,807,989	1,986,260
Common stock	1,727,859	33,896
Additional paid-in capital	117,737	117,737
Retained earnings (deficit)	904,472	749,674
Total Steel Dynamics, Inc. equity	2,750,068	901,307
Total equity	2,750,068	901,307
Total liabilities and equity	5,096,596	3,261,455
Reportable legal entity | Combined Non-Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	14,360	13,142	11,535	30,989
Accounts receivable, net	44,696	32,018
Inventories	94,916	91,199
Other current assets	6,577	3,259
Total current assets	160,549	139,618
Property, plant and equipment, net	297,505	562,843
Other assets, including investments in subs	6,635	8,092
Total assets	464,689	710,553
Accounts payable	98,886	60,987
Accrued expenses	11,695	10,694
Current maturities of long-term debt	73,767	52,163
Total current liabilities	184,348	123,844
Long-term debt	158,665	204,385
Other liabilities	28,719	34,895
Redeemable noncontrolling interests	126,340	116,514
Common stock	18,121	18,121
Additional paid-in capital	635,156	552,946
Retained earnings (deficit)	(569,172)	(288,684)
Total Steel Dynamics, Inc. equity	84,105	282,383
Noncontrolling interests	(117,488)	(51,468)
Total equity	(33,383)	230,915
Total liabilities and equity	464,689	710,553
Consolidating Adjustments
Condensed Consolidating Balance Sheets
Accounts receivable, net	(640,213)	(495,816)
Inventories	(2,263)	(7,855)
Other current assets	(18,469)	(18,755)
Total current assets	(660,945)	(522,426)
Property, plant and equipment, net	(2,214)	(2,419)
Other assets, including investments in subs	(3,834,607)	(2,545,209)
Total assets	(4,497,766)	(3,070,054)
Accounts payable	(110,384)	(64,716)
Accrued expenses	(76,163)	(49,229)
Current maturities of long-term debt	(41,157)	(26,440)
Total current liabilities	(227,704)	(140,385)
Long-term debt	(123,943)	(163,773)
Other liabilities	(1,311,946)	(1,582,206)
Common stock	(1,745,980)	(52,017)
Additional paid-in capital	(752,893)	(670,683)
Retained earnings (deficit)	(335,300)	(460,990)
Total Steel Dynamics, Inc. equity	(2,834,173)	(1,183,690)
Total equity	(2,834,173)	(1,183,690)
Total liabilities and equity	$ (4,497,766)	$ (3,070,054)