Condensed Consolidating Information (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013
Condensed Consolidating Balance Sheets
Ownership interest in subsidiaries (as a percent)	100.00%
Cash and equivalents	$ 419,401	$ 160,982	$ 361,363	$ 357,490	$ 342,919	$ 395,156
Accounts receivable, net	816,522		902,825
Inventories	1,292,069		1,618,419
Other current assets	63,400		91,158
Total current assets	2,591,392		2,973,765
Property, plant and equipment, net	3,047,401		3,123,906
Intangible assets, net	358,402		370,669
Goodwill	741,898		745,158
Other assets, including investments in subs	87,670		97,529
Total assets	6,826,763		7,311,027	5,985,202
Accounts payable	446,698		511,056
Accrued expenses	216,815		293,066
Current maturities of long-term debt	35,075		46,460
Total current liabilities	698,588		850,582
Long-term debt	2,619,574		2,977,706
Other liabilities	583,901		560,872
Redeemable noncontrolling interests	125,972		126,340
Common stock	636		635
Treasury stock	(396,491)		(398,898)
Additional paid-in capital	1,099,669		1,083,435
Retained earnings (deficit)	2,223,599		2,227,843
Total Steel Dynamics, Inc. equity	2,927,413		2,913,015
Noncontrolling interests	(128,685)		(117,488)
Total equity	2,798,728		2,795,527
Total liabilities and equity	6,826,763		7,311,027
Reportable legal entity | Parent
Condensed Consolidating Balance Sheets
Cash and equivalents	351,996		265,313	297,225		320,866
Accounts receivable, net	262,708		321,493
Inventories	590,104		662,970
Other current assets	69,807		94,634
Total current assets	1,274,615		1,344,410
Property, plant and equipment, net	979,749		1,002,407
Other assets, including investments in subs	3,573,407		3,900,691
Total assets	5,827,771		6,247,508
Accounts payable	140,249		151,517
Accrued expenses	126,025		191,433
Current maturities of long-term debt	13,097		13,073
Total current liabilities	279,371		356,023
Long-term debt	2,585,806		2,942,360
Other liabilities	35,181		36,110
Common stock	636		635
Treasury stock	(396,491)		(398,898)
Additional paid-in capital	1,099,669		1,083,435
Retained earnings (deficit)	2,223,599		2,227,843
Total Steel Dynamics, Inc. equity	2,927,413		2,913,015
Total equity	2,927,413		2,913,015
Total liabilities and equity	5,827,771		6,247,508
Reportable legal entity | Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	55,586		81,690	50,461		61,148
Accounts receivable, net	1,182,293		1,176,849
Inventories	658,048		862,796
Other current assets	7,343		8,416
Total current assets	1,903,270		2,129,751
Property, plant and equipment, net	1,769,536		1,826,208
Intangible assets, net	358,402		370,669
Goodwill	741,898		745,158
Other assets, including investments in subs	23,085		24,810
Total assets	4,796,191		5,096,596
Accounts payable	314,074		371,037
Accrued expenses	168,859		166,101
Current maturities of long-term debt	809		777
Total current liabilities	483,742		537,915
Long-term debt	224		624
Other liabilities	1,505,382		1,807,989
Common stock	1,727,859		1,727,859
Additional paid-in capital	117,737		117,737
Retained earnings (deficit)	961,247		904,472
Total Steel Dynamics, Inc. equity	2,806,843		2,750,068
Total equity	2,806,843		2,750,068
Total liabilities and equity	4,796,191		5,096,596
Reportable legal entity | Combined Non-Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	11,819		14,360	$ 9,804		$ 13,142
Accounts receivable, net	36,919		44,696
Inventories	47,494		94,916
Other current assets	4,436		6,577
Total current assets	100,668		160,549
Property, plant and equipment, net	300,229		297,505
Other assets, including investments in subs	6,491		6,635
Total assets	407,388		464,689
Accounts payable	89,416		98,886
Accrued expenses	6,484		11,695
Current maturities of long-term debt	61,445		73,767
Total current liabilities	157,345		184,348
Long-term debt	167,639		158,665
Other liabilities	35,490		28,719
Redeemable noncontrolling interests	125,972		126,340
Common stock	18,121		18,121
Additional paid-in capital	641,858		635,156
Retained earnings (deficit)	(610,352)		(569,172)
Total Steel Dynamics, Inc. equity	49,627		84,105
Noncontrolling interests	(128,685)		(117,488)
Total equity	(79,058)		(33,383)
Total liabilities and equity	407,388		464,689
Consolidating Adjustments
Condensed Consolidating Balance Sheets
Accounts receivable, net	(665,398)		(640,213)
Inventories	(3,577)		(2,263)
Other current assets	(18,186)		(18,469)
Total current assets	(687,161)		(660,945)
Property, plant and equipment, net	(2,113)		(2,214)
Other assets, including investments in subs	(3,515,313)		(3,834,607)
Total assets	(4,204,587)		(4,497,766)
Accounts payable	(97,041)		(110,384)
Accrued expenses	(84,553)		(76,163)
Current maturities of long-term debt	(40,276)		(41,157)
Total current liabilities	(221,870)		(227,704)
Long-term debt	(134,095)		(123,943)
Other liabilities	(992,152)		(1,311,946)
Common stock	(1,745,980)		(1,745,980)
Additional paid-in capital	(759,595)		(752,893)
Retained earnings (deficit)	(350,895)		(335,300)
Total Steel Dynamics, Inc. equity	(2,856,470)		(2,834,173)
Total equity	(2,856,470)		(2,834,173)
Total liabilities and equity	$ (4,204,587)		$ (4,497,766)