Condensed Consolidating Information - Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Condensed Consolidating Balance Sheets
Cash and equivalents	$ 1,051,489	$ 1,052,666	$ 727,032	$ 473,790	$ 419,401	$ 361,363
Accounts receivable, net	779,459		613,605
Inventories	1,275,575		1,149,390
Other current assets	30,121		47,914
Total current assets	3,136,644		2,537,941
Property, plant and equipment, net	2,928,226		2,951,210
Intangible assets, net	291,814		278,960
Goodwill	399,867		397,470
Other assets, including investments in subs	31,011		36,501
Total assets	6,787,562		6,202,082	6,842,541
Accounts payable	437,836		283,355
Accrued expenses	340,122		235,255
Current maturities of long-term debt	16,155		16,680
Total current liabilities	794,113		535,290
Long-term debt	2,570,837		2,577,976
Other liabilities	470,910		417,365
Redeemable noncontrolling interests	126,340		126,340
Common stock	640		638
Treasury stock	(392,051)		(396,455)
Additional paid-in capital	1,132,365		1,110,253
Retained earnings (deficit)	2,224,963		1,965,291
Total Steel Dynamics, Inc. equity	2,965,917		2,679,727
Noncontrolling interests	(140,555)		(134,616)
Total equity	2,825,362		2,545,111
Total liabilities and equity	6,787,562		6,202,082
Parent
Condensed Consolidating Balance Sheets
Cash and equivalents	972,610		636,877	410,170		265,313
Accounts receivable, net	238,290		200,094
Inventories	588,583		539,963
Other current assets	17,109		21,654
Total current assets	1,816,592		1,398,588
Property, plant and equipment, net	921,850		958,212
Other assets, including investments in subs	2,916,319		2,941,710
Total assets	5,654,761		5,298,510
Accounts payable	171,302		100,751
Accrued expenses	229,039		141,552
Current maturities of long-term debt	13,161		13,122
Total current liabilities	413,502		255,425
Long-term debt	2,541,145		2,546,606
Other liabilities	(265,803)		(183,248)
Common stock	640		638
Treasury stock	(392,051)		(396,455)
Additional paid-in capital	1,132,365		1,110,253
Retained earnings (deficit)	2,224,963		1,965,291
Total Steel Dynamics, Inc. equity	2,965,917		2,679,727
Total equity	2,965,917		2,679,727
Total liabilities and equity	5,654,761		5,298,510
Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	61,054		81,976	52,752		81,690
Accounts receivable, net	1,248,565		1,056,285
Inventories	634,942		573,924
Other current assets	10,598		25,415
Total current assets	1,955,159		1,737,600
Property, plant and equipment, net	1,684,925		1,703,932
Intangible assets, net	259,231		278,960
Goodwill	392,678		397,470
Other assets, including investments in subs	8,023		10,040
Total assets	4,300,016		4,128,002
Accounts payable	274,873		183,344
Accrued expenses	212,493		185,873
Current maturities of long-term debt	700		700
Total current liabilities	488,066		369,917
Long-term debt			361
Other liabilities	1,155,611		1,342,541
Common stock	1,727,859		1,727,859
Additional paid-in capital	128,076		117,737
Retained earnings (deficit)	800,404		569,587
Total Steel Dynamics, Inc. equity	2,656,339		2,415,183
Total equity	2,656,339		2,415,183
Total liabilities and equity	4,300,016		4,128,002
Combined Non-Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	17,825		8,179	$ 10,868		$ 14,360
Accounts receivable, net	41,667		29,775
Inventories	59,837		35,004
Other current assets	4,373		1,676
Total current assets	123,702		74,634
Property, plant and equipment, net	323,308		291,077
Intangible assets, net	32,583
Goodwill	7,189
Other assets, including investments in subs	6,411		6,137
Total assets	493,193		371,848
Accounts payable	80,690		68,948
Accrued expenses	8,068		4,779
Current maturities of long-term debt	32,248		24,975
Total current liabilities	121,006		98,702
Long-term debt	171,622		177,897
Other liabilities	61,932		63,020
Redeemable noncontrolling interests	126,340		126,340
Common stock	18,120		18,120
Additional paid-in capital	773,793		646,787
Retained earnings (deficit)	(639,065)		(624,402)
Total Steel Dynamics, Inc. equity	152,848		40,505
Noncontrolling interests	(140,555)		(134,616)
Total equity	12,293		(94,111)
Total liabilities and equity	493,193		371,848
Consolidating Adjustments
Condensed Consolidating Balance Sheets
Accounts receivable, net	(749,063)		(672,549)
Inventories	(7,787)		499
Other current assets	(1,959)		(831)
Total current assets	(758,809)		(672,881)
Property, plant and equipment, net	(1,857)		(2,011)
Other assets, including investments in subs	(2,899,742)		(2,921,386)
Total assets	(3,660,408)		(3,596,278)
Accounts payable	(89,029)		(69,688)
Accrued expenses	(109,478)		(96,949)
Current maturities of long-term debt	(29,954)		(22,117)
Total current liabilities	(228,461)		(188,754)
Long-term debt	(141,930)		(146,888)
Other liabilities	(480,830)		(804,948)
Common stock	(1,745,979)		(1,745,979)
Additional paid-in capital	(901,869)		(764,524)
Retained earnings (deficit)	(161,339)		54,815
Total Steel Dynamics, Inc. equity	(2,809,187)		(2,455,688)
Total equity	(2,809,187)		(2,455,688)
Total liabilities and equity	$ (3,660,408)		$ (3,596,278)