Condensed Consolidating Information (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Condensed Consolidating Balance Sheets
Ownership interest in subsidiaries (as a percent)	100.00%
Cash and equivalents	$ 160,982	$ 361,363	$ 342,919	$ 395,156
Accounts receivable, net	769,741	902,825
Inventories	1,453,242	1,618,419
Other current assets	68,260	91,158
Total current assets	2,452,225	2,973,765
Property, plant and equipment, net	3,085,587	3,123,906
Intangible assets, net	364,620	370,669
Goodwill	743,528	745,158
Other assets, including investments in subs	89,878	97,529
Total assets	6,735,838	7,311,027	5,913,857
Accounts payable	376,962	511,056
Accrued expenses	210,645	293,066
Current maturities of long-term debt	30,727	46,460
Total current liabilities	618,334	850,582
Long-term debt	2,623,444	2,977,706
Other liabilities	570,967	560,872
Redeemable noncontrolling interests	126,340	126,340
Common stock	635	635
Treasury stock	(396,517)	(398,898)
Additional paid-in capital	1,088,605	1,083,435
Retained earnings (deficit)	2,225,354	2,227,843
Total Steel Dynamics, Inc. equity	2,918,077	2,913,015
Noncontrolling interests	(121,324)	(117,488)
Total equity	2,796,753	2,795,527
Total liabilities and equity	6,735,838	7,311,027
Reportable legal entity | Parent
Condensed Consolidating Balance Sheets
Cash and equivalents	130,541	265,313	281,376	320,866
Accounts receivable, net	258,508	321,493
Inventories	611,529	662,970
Other current assets	72,189	94,634
Total current assets	1,072,767	1,344,410
Property, plant and equipment, net	989,601	1,002,407
Other assets, including investments in subs	3,745,496	3,900,691
Total assets	5,807,864	6,247,508
Accounts payable	117,240	151,517
Accrued expenses	133,295	191,433
Current maturities of long-term debt	13,085	13,073
Total current liabilities	263,620	356,023
Long-term debt	2,589,085	2,942,360
Other liabilities	37,082	36,110
Common stock	635	635
Treasury stock	(396,517)	(398,898)
Additional paid-in capital	1,088,605	1,083,435
Retained earnings (deficit)	2,225,354	2,227,843
Total Steel Dynamics, Inc. equity	2,918,077	2,913,015
Total equity	2,918,077	2,913,015
Total liabilities and equity	5,807,864	6,247,508
Reportable legal entity | Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	17,058	81,690	48,071	61,148
Accounts receivable, net	1,129,116	1,176,849
Inventories	759,470	862,796
Other current assets	9,213	8,416
Total current assets	1,914,857	2,129,751
Property, plant and equipment, net	1,796,992	1,826,208
Intangible assets, net	364,620	370,669
Goodwill	743,528	745,158
Other assets, including investments in subs	23,871	24,810
Total assets	4,843,868	5,096,596
Accounts payable	262,723	371,037
Accrued expenses	149,157	166,101
Current maturities of long-term debt	795	777
Total current liabilities	412,675	537,915
Long-term debt	224	624
Other liabilities	1,655,611	1,807,989
Common stock	1,727,860	1,727,859
Additional paid-in capital	117,737	117,737
Retained earnings (deficit)	929,761	904,472
Total Steel Dynamics, Inc. equity	2,775,358	2,750,068
Total equity	2,775,358	2,750,068
Total liabilities and equity	4,843,868	5,096,596
Reportable legal entity | Combined Non-Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	13,383	14,360	13,472	13,142
Accounts receivable, net	24,017	44,696
Inventories	82,347	94,916
Other current assets	5,243	6,577
Total current assets	124,990	160,549
Property, plant and equipment, net	301,158	297,505
Other assets, including investments in subs	6,524	6,635
Total assets	432,672	464,689
Accounts payable	89,959	98,886
Accrued expenses	7,649	11,695
Current maturities of long-term debt	57,997	73,767
Total current liabilities	155,605	184,348
Long-term debt	167,621	158,665
Other liabilities	31,706	28,719
Redeemable noncontrolling interests	126,340	126,340
Common stock	18,121	18,121
Additional paid-in capital	635,335	635,156
Retained earnings (deficit)	(580,732)	(569,172)
Total Steel Dynamics, Inc. equity	72,724	84,105
Noncontrolling interests	(121,324)	(117,488)
Total equity	(48,600)	(33,383)
Total liabilities and equity	432,672	464,689
Consolidating Adjustments
Condensed Consolidating Balance Sheets
Accounts receivable, net	(641,900)	(640,213)
Inventories	(104)	(2,263)
Other current assets	(18,385)	(18,469)
Total current assets	(660,389)	(660,945)
Property, plant and equipment, net	(2,164)	(2,214)
Other assets, including investments in subs	(3,686,013)	(3,834,607)
Total assets	(4,348,566)	(4,497,766)
Accounts payable	(92,960)	(110,384)
Accrued expenses	(79,456)	(76,163)
Current maturities of long-term debt	(41,150)	(41,157)
Total current liabilities	(213,566)	(227,704)
Long-term debt	(133,486)	(123,943)
Other liabilities	(1,153,432)	(1,311,946)
Common stock	(1,745,981)	(1,745,980)
Additional paid-in capital	(753,072)	(752,893)
Retained earnings (deficit)	(349,029)	(335,300)
Total Steel Dynamics, Inc. equity	(2,848,082)	(2,834,173)
Total equity	(2,848,082)	(2,834,173)
Total liabilities and equity	$ (4,348,566)	$ (4,497,766)