Condensed Consolidating Information - Balance Sheets (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Condensed Consolidating Balance Sheets
Ownership interest in subsidiaries (as a percent)	100.00%
Cash and equivalents	$ 976,951	$ 727,032	$ 160,982	$ 361,363
Accounts receivable, net	689,201	613,605
Inventories	1,066,823	1,149,390
Other current assets	41,258	47,914
Total current assets	2,774,233	2,537,941
Property, plant and equipment, net	2,914,186	2,951,210
Intangible assets, net	272,528	278,960
Goodwill	395,872	397,470
Other assets, including investments in subs	34,823	36,501
Total assets	6,391,642	6,202,082
Accounts payable	413,575	283,355
Accrued expenses	236,701	235,255
Current maturities of long-term debt	22,770	16,680
Total current liabilities	673,046	535,290
Long-term debt	2,575,528	2,577,976
Other liabilities	435,562	417,365
Redeemable noncontrolling interests	126,340	126,340
Common stock	639	638
Treasury stock	(392,348)	(396,455)
Additional paid-in capital	1,115,008	1,110,253
Retained earnings (deficit)	1,993,910	1,965,291
Total Steel Dynamics, Inc. equity	2,717,209	2,679,727
Noncontrolling interests	(136,043)	(134,616)
Total equity	2,581,166	2,545,111
Total liabilities and equity	6,391,642	6,202,082
Reportable legal entity | Parent
Condensed Consolidating Balance Sheets
Cash and equivalents	887,690	636,877	130,541	265,313
Accounts receivable, net	219,883	200,094
Inventories	516,467	539,963
Other current assets	19,602	21,654
Total current assets	1,643,642	1,398,588
Property, plant and equipment, net	948,732	958,212
Other assets, including investments in subs	2,778,142	2,941,710
Total assets	5,370,516	5,298,510
Accounts payable	138,423	100,751
Accrued expenses	146,230	141,552
Current maturities of long-term debt	13,134	13,122
Total current liabilities	297,787	255,425
Long-term debt	2,544,789	2,546,606
Other liabilities	(189,269)	(183,248)
Common stock	639	638
Treasury stock	(392,348)	(396,455)
Additional paid-in capital	1,115,008	1,110,253
Retained earnings (deficit)	1,993,910	1,965,291
Total Steel Dynamics, Inc. equity	2,717,209	2,679,727
Total equity	2,717,209	2,679,727
Total liabilities and equity	5,370,516	5,298,510
Reportable legal entity | Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	75,082	81,976	17,058	81,690
Accounts receivable, net	1,128,020	1,056,285
Inventories	518,657	573,924
Other current assets	20,902	25,415
Total current assets	1,742,661	1,737,600
Property, plant and equipment, net	1,677,310	1,703,932
Intangible assets, net	272,528	278,960
Goodwill	395,872	397,470
Other assets, including investments in subs	10,003	10,040
Total assets	4,098,374	4,128,002
Accounts payable	281,926	183,344
Accrued expenses	185,090	185,873
Current maturities of long-term debt	700	700
Total current liabilities	467,716	369,917
Long-term debt		361
Other liabilities	1,173,238	1,342,541
Common stock	1,727,859	1,727,859
Additional paid-in capital	117,737	117,737
Retained earnings (deficit)	611,824	569,587
Total Steel Dynamics, Inc. equity	2,457,420	2,415,183
Total equity	2,457,420	2,415,183
Total liabilities and equity	4,098,374	4,128,002
Reportable legal entity | Combined Non-Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	14,179	8,179	$ 13,383	$ 14,360
Accounts receivable, net	32,979	29,775
Inventories	34,105	35,004
Other current assets	2,128	1,676
Total current assets	83,391	74,634
Property, plant and equipment, net	290,103	291,077
Other assets, including investments in subs	6,128	6,137
Total assets	379,622	371,848
Accounts payable	66,679	68,948
Accrued expenses	4,590	4,779
Current maturities of long-term debt	34,034	24,975
Total current liabilities	105,303	98,702
Long-term debt	171,390	177,897
Other liabilities	70,280	63,020
Redeemable noncontrolling interests	126,340	126,340
Common stock	18,120	18,120
Additional paid-in capital	653,787	646,787
Retained earnings (deficit)	(629,555)	(624,402)
Total Steel Dynamics, Inc. equity	42,352	40,505
Noncontrolling interests	(136,043)	(134,616)
Total equity	(93,691)	(94,111)
Total liabilities and equity	379,622	371,848
Consolidating Adjustments
Condensed Consolidating Balance Sheets
Accounts receivable, net	(691,681)	(672,549)
Inventories	(2,406)	499
Other current assets	(1,374)	(831)
Total current assets	(695,461)	(672,881)
Property, plant and equipment, net	(1,959)	(2,011)
Other assets, including investments in subs	(2,759,450)	(2,921,386)
Total assets	(3,456,870)	(3,596,278)
Accounts payable	(73,453)	(69,688)
Accrued expenses	(99,209)	(96,949)
Current maturities of long-term debt	(25,098)	(22,117)
Total current liabilities	(197,760)	(188,754)
Long-term debt	(140,651)	(146,888)
Other liabilities	(618,687)	(804,948)
Common stock	(1,745,979)	(1,745,979)
Additional paid-in capital	(771,524)	(764,524)
Retained earnings (deficit)	17,731	54,815
Total Steel Dynamics, Inc. equity	(2,499,772)	(2,455,688)
Total equity	(2,499,772)	(2,455,688)
Total liabilities and equity	$ (3,456,870)	$ (3,596,278)