Condensed Consolidating Information (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Condensed Consolidating Balance Sheets
Ownership interest in subsidiaries (as a percent)	100.00%
Cash and equivalents	$ 473,790	$ 419,401	$ 361,363	$ 159,591	$ 357,490	$ 395,156
Accounts receivable, net	799,025		902,825
Inventories	1,321,397		1,618,419
Other current assets	57,583		91,158
Total current assets	2,651,795		2,973,765
Property, plant and equipment, net	3,013,659		3,123,906
Intangible assets, net	353,561		370,669
Goodwill	740,243		745,158
Other assets, including investments in subs	83,283		97,529
Total assets	6,842,541		7,311,027	7,578,283
Accounts payable	386,370		511,056
Accrued expenses	259,904		293,066
Current maturities of long-term debt	31,584		46,460
Total current liabilities	677,858		850,582
Long-term debt	2,615,819		2,977,706
Other liabilities	593,079		560,872
Redeemable noncontrolling interests	126,340		126,340
Common stock	636		635
Treasury stock	(396,473)		(398,898)
Additional paid-in capital	1,104,832		1,083,435
Retained earnings (deficit)	2,250,901		2,227,843
Total Steel Dynamics, Inc. equity	2,959,896		2,913,015
Noncontrolling interests	(130,451)		(117,488)
Total equity	2,829,445		2,795,527
Total liabilities and equity	6,842,541		7,311,027
Reportable legal entity | Parent
Condensed Consolidating Balance Sheets
Cash and equivalents	410,170		265,313	74,925		320,866
Accounts receivable, net	235,529		321,493
Inventories	613,697		662,970
Other current assets	61,961		94,634
Total current assets	1,321,357		1,344,410
Property, plant and equipment, net	966,388		1,002,407
Other assets, including investments in subs	3,570,277		3,900,691
Total assets	5,858,022		6,247,508
Accounts payable	122,112		151,517
Accrued expenses	156,735		191,433
Current maturities of long-term debt	13,109		13,073
Total current liabilities	291,956		356,023
Long-term debt	2,582,524		2,942,360
Other liabilities	23,645		36,110
Common stock	636		635
Treasury stock	(396,473)		(398,898)
Additional paid-in capital	1,104,833		1,083,435
Retained earnings (deficit)	2,250,901		2,227,843
Total Steel Dynamics, Inc. equity	2,959,897		2,913,015
Total equity	2,959,897		2,913,015
Total liabilities and equity	5,858,022		6,247,508
Reportable legal entity | Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	52,752		81,690	70,695		61,148
Accounts receivable, net	1,199,733		1,176,849
Inventories	673,646		862,796
Other current assets	11,830		8,416
Total current assets	1,937,961		2,129,751
Property, plant and equipment, net	1,754,945		1,826,208
Intangible assets, net	353,561		370,669
Goodwill	740,243		745,158
Other assets, including investments in subs	22,075		24,810
Total assets	4,808,785		5,096,596
Accounts payable	273,957		371,037
Accrued expenses	186,923		166,101
Current maturities of long-term debt	700		777
Total current liabilities	461,580		537,915
Long-term debt	347		624
Other liabilities	1,505,018		1,807,989
Common stock	1,727,859		1,727,859
Additional paid-in capital	117,737		117,737
Retained earnings (deficit)	996,244		904,472
Total Steel Dynamics, Inc. equity	2,841,840		2,750,068
Total equity	2,841,840		2,750,068
Total liabilities and equity	4,808,785		5,096,596
Reportable legal entity | Combined Non-Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	10,868		14,360	$ 13,971		$ 13,142
Accounts receivable, net	40,496		44,696
Inventories	36,150		94,916
Other current assets	1,150		6,577
Total current assets	88,664		160,549
Property, plant and equipment, net	294,388		297,505
Other assets, including investments in subs	6,454		6,635
Total assets	389,506		464,689
Accounts payable	79,370		98,886
Accrued expenses	5,681		11,695
Current maturities of long-term debt	57,253		73,767
Total current liabilities	142,304		184,348
Long-term debt	163,728		158,665
Other liabilities	34,545		28,719
Redeemable noncontrolling interests	126,340		126,340
Common stock	18,121		18,121
Additional paid-in capital	650,858		635,156
Retained earnings (deficit)	(615,939)		(569,172)
Total Steel Dynamics, Inc. equity	53,040		84,105
Noncontrolling interests	(130,451)		(117,488)
Total equity	(77,411)		(33,383)
Total liabilities and equity	389,506		464,689
Consolidating Adjustments
Condensed Consolidating Balance Sheets
Accounts receivable, net	(676,733)		(640,213)
Inventories	(2,096)		(2,263)
Other current assets	(17,358)		(18,469)
Total current assets	(696,187)		(660,945)
Property, plant and equipment, net	(2,062)		(2,214)
Other assets, including investments in subs	(3,515,523)		(3,834,607)
Total assets	(4,213,772)		(4,497,766)
Accounts payable	(89,069)		(110,384)
Accrued expenses	(89,435)		(76,163)
Current maturities of long-term debt	(39,478)		(41,157)
Total current liabilities	(217,982)		(227,704)
Long-term debt	(130,780)		(123,943)
Other liabilities	(970,129)		(1,311,946)
Common stock	(1,745,980)		(1,745,980)
Additional paid-in capital	(768,596)		(752,893)
Retained earnings (deficit)	(380,305)		(335,300)
Total Steel Dynamics, Inc. equity	(2,894,881)		(2,834,173)
Total equity	(2,894,881)		(2,834,173)
Total liabilities and equity	$ (4,213,772)		$ (4,497,766)