Condensed Consolidating Information - Balance Sheets (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014
Condensed Consolidating Balance Sheets
Ownership interest in subsidiaries (as a percent)	100.00%
Cash and equivalents	$ 1,052,666	$ 976,951	$ 727,032	$ 419,401	$ 160,982	$ 361,363
Accounts receivable, net	794,003		613,605
Inventories	1,175,716		1,149,390
Other current assets	28,072		47,914
Total current assets	3,050,457		2,537,941
Property, plant and equipment, net	2,885,844		2,951,210
Intangible assets, net	265,476		278,960
Goodwill	394,275		397,470
Other assets, including investments in subs	33,624		36,501
Total assets	6,629,676		6,202,082	6,826,763
Accounts payable	461,267		283,355
Accrued expenses	299,910		235,255
Current maturities of long-term debt	18,047		16,680
Total current liabilities	779,224		535,290
Long-term debt	2,573,186		2,577,976
Other liabilities	452,660		417,365
Redeemable noncontrolling interests	126,340		126,340
Common stock	639		638
Treasury stock	(392,050)		(396,455)
Additional paid-in capital	1,125,519		1,110,253
Retained earnings (deficit)	2,101,729		1,965,291
Total Steel Dynamics, Inc. equity	2,835,837		2,679,727
Noncontrolling interests	(137,571)		(134,616)
Total equity	2,698,266		2,545,111
Total liabilities and equity	6,629,676		6,202,082
Reportable legal entity | Parent
Condensed Consolidating Balance Sheets
Cash and equivalents	947,018		636,877	351,996		265,313
Accounts receivable, net	263,090		200,094
Inventories	561,567		539,963
Other current assets	15,660		21,654
Total current assets	1,787,335		1,398,588
Property, plant and equipment, net	932,067		958,212
Other assets, including investments in subs	2,811,879		2,941,710
Total assets	5,531,281		5,298,510
Accounts payable	169,025		100,751
Accrued expenses	198,810		141,552
Current maturities of long-term debt	13,147		13,122
Total current liabilities	380,982		255,425
Long-term debt	2,542,969		2,546,606
Other liabilities	(228,508)		(183,248)
Common stock	640		638
Treasury stock	(392,050)		(396,455)
Additional paid-in capital	1,125,519		1,110,253
Retained earnings (deficit)	2,101,729		1,965,291
Total Steel Dynamics, Inc. equity	2,835,838		2,679,727
Total equity	2,835,838		2,679,727
Total liabilities and equity	5,531,281		5,298,510
Reportable legal entity | Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	88,893		81,976	55,586		81,690
Accounts receivable, net	1,228,150		1,056,285
Inventories	583,995		573,924
Other current assets	12,345		25,415
Total current assets	1,913,383		1,737,600
Property, plant and equipment, net	1,668,643		1,703,932
Intangible assets, net	265,476		278,960
Goodwill	394,275		397,470
Other assets, including investments in subs	8,986		10,040
Total assets	4,250,763		4,128,002
Accounts payable	306,782		183,344
Accrued expenses	199,976		185,873
Current maturities of long-term debt	700		700
Total current liabilities	507,458		369,917
Long-term debt			361
Other liabilities	1,188,931		1,342,541
Common stock	1,727,859		1,727,859
Additional paid-in capital	117,737		117,737
Retained earnings (deficit)	708,778		569,587
Total Steel Dynamics, Inc. equity	2,554,374		2,415,183
Total equity	2,554,374		2,415,183
Total liabilities and equity	4,250,763		4,128,002
Reportable legal entity | Combined Non-Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	16,755		8,179	$ 11,819		$ 14,360
Accounts receivable, net	30,318		29,775
Inventories	37,484		35,004
Other current assets	1,753		1,676
Total current assets	86,310		74,634
Property, plant and equipment, net	287,042		291,077
Other assets, including investments in subs	6,257		6,137
Total assets	379,609		371,848
Accounts payable	74,445		68,948
Accrued expenses	5,257		4,779
Current maturities of long-term debt	28,401		24,975
Total current liabilities	108,103		98,702
Long-term debt	174,689		177,897
Other liabilities	70,488		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)	(634,347)		(624,402)
Total Steel Dynamics, Inc. equity	37,560		40,505
Noncontrolling interests	(137,571)		(134,616)
Total equity	(100,011)		(94,111)
Total liabilities and equity	379,609		371,848
Consolidating Adjustments
Condensed Consolidating Balance Sheets
Accounts receivable, net	(727,555)		(672,549)
Inventories	(7,330)		499
Other current assets	(1,686)		(831)
Total current assets	(736,571)		(672,881)
Property, plant and equipment, net	(1,908)		(2,011)
Other assets, including investments in subs	(2,793,498)		(2,921,386)
Total assets	(3,531,977)		(3,596,278)
Accounts payable	(88,985)		(69,688)
Accrued expenses	(104,133)		(96,949)
Current maturities of long-term debt	(24,201)		(22,117)
Total current liabilities	(217,319)		(188,754)
Long-term debt	(144,472)		(146,888)
Other liabilities	(578,251)		(804,948)
Common stock	(1,745,980)		(1,745,979)
Additional paid-in capital	(771,524)		(764,524)
Retained earnings (deficit)	(74,431)		54,815
Total Steel Dynamics, Inc. equity	(2,591,935)		(2,455,688)
Total equity	(2,591,935)		(2,455,688)
Total liabilities and equity	$ (3,531,977)		$ (3,596,278)