Condensed Consolidating Information (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010
Condensed Consolidating Financial Statements
Ownership interest in subsidiaries (as a percent)	100.00%
Cash and equivalents	$ 420,938	$ 357,809	$ 390,761	$ 277,691	$ 160,810	$ 186,513
Investments in short-term commercial paper	9,998		84,830
Accounts receivable, net	715,440		722,791
Inventories	1,252,153		1,199,584
Other current assets	59,815		57,292
Total current assets	2,458,344		2,455,258
Property, plant and equipment, net	2,208,660		2,193,745
Intangible assets, net	433,559		450,893
Goodwill	741,817		745,066
Other assets, including investments in subs	129,000		134,264
Total assets	5,971,380		5,979,226	5,900,453
Accounts payable	393,174		420,824
Accrued expenses	186,363		235,515
Current maturities of long-term debt	439,912		444,078
Total current liabilities	1,019,449		1,100,417
Long-term debt	1,928,097		1,936,022
Other liabilities	592,274		572,193
Redeemable noncontrolling interest	77,424		70,694
Common stock	636		636
Treasury stock	(720,481)		(722,653)
Additional paid-in capital	1,033,086		1,026,157
Retained earnings (deficit)	2,058,108		2,011,801
Total Steel Dynamics, Inc. equity	2,371,349		2,315,941
Noncontrolling interests	(17,213)		(16,041)
Total equity	2,354,136		2,299,900
Total liabilities and equity	5,971,380		5,979,226
Parent
Condensed Consolidating Financial Statements
Cash and equivalents	304,807		301,073	264,121		173,563
Investments in short-term commercial paper	9,998		84,830
Accounts receivable, net	302,173		319,995
Inventories	648,242		584,079
Other current assets	71,774		83,116
Total current assets	1,336,994		1,373,093
Property, plant and equipment, net	1,028,099		1,059,011
Other assets, including investments in subs	2,819,033		2,791,718
Total assets	5,184,126		5,223,822
Accounts payable	123,796		155,220
Accrued expenses	110,034		157,868
Current maturities of long-term debt	434,498		439,199
Total current liabilities	668,328		752,287
Long-term debt	1,898,084		1,905,199
Other liabilities	246,365		250,395
Common stock	636		636
Treasury stock	(720,481)		(722,653)
Additional paid-in capital	1,033,086		1,026,157
Retained earnings (deficit)	2,058,108		2,011,801
Total Steel Dynamics, Inc. equity	2,371,349		2,315,941
Total equity	2,371,349		2,315,941
Total liabilities and equity	5,184,126		5,223,822
Guarantors
Condensed Consolidating Financial Statements
Cash and equivalents	105,248		58,699	9,170		10,628
Accounts receivable, net	767,638		726,192
Inventories	522,961		533,022
Other current assets	7,755		4,119
Total current assets	1,403,602		1,322,032
Property, plant and equipment, net	656,804		660,048
Intangible assets, net	433,559		450,893
Goodwill	741,817		745,066
Other assets, including investments in subs	31,869		33,507
Total assets	3,267,651		3,211,546
Accounts payable	268,860		273,049
Accrued expenses	100,385		98,498
Current maturities of long-term debt	300		300
Total current liabilities	369,545		371,847
Other liabilities	2,146,367		2,132,778
Common stock	33,896		33,896
Additional paid-in capital	117,737		117,737
Retained earnings (deficit)	600,106		555,288
Total Steel Dynamics, Inc. equity	751,739		706,921
Total equity	751,739		706,921
Total liabilities and equity	3,267,651		3,211,546
Combined Non-Guarantors
Condensed Consolidating Financial Statements
Cash and equivalents	10,883		30,989	4,400		2,322
Accounts receivable, net	11,230		8,971
Inventories	84,899		85,046
Other current assets	1,910		2,304
Total current assets	108,922		127,310
Property, plant and equipment, net	526,482		477,514
Other assets, including investments in subs	9,417		9,853
Total assets	644,821		614,677
Accounts payable	39,208		23,195
Accrued expenses	11,665		10,012
Current maturities of long-term debt	25,608		53,551
Total current liabilities	76,481		86,758
Long-term debt	215,034		169,797
Other liabilities	41,360		51,625
Redeemable noncontrolling interest	77,424		70,694
Common stock	18,121		18,121
Additional paid-in capital	384,888		347,151
Retained earnings (deficit)	(151,274)		(113,428)
Total Steel Dynamics, Inc. equity	251,735		251,844
Noncontrolling interests	(17,213)		(16,041)
Total equity	234,522		235,803
Total liabilities and equity	644,821		614,677
Consolidating Adjustments
Condensed Consolidating Financial Statements
Accounts receivable, net	(365,601)		(332,367)
Inventories	(3,949)		(2,563)
Other current assets	(21,624)		(32,247)
Total current assets	(391,174)		(367,177)
Property, plant and equipment, net	(2,725)		(2,828)
Other assets, including investments in subs	(2,731,319)		(2,700,814)
Total assets	(3,125,218)		(3,070,819)
Accounts payable	(38,690)		(30,640)
Accrued expenses	(35,721)		(30,863)
Current maturities of long-term debt	(20,494)		(48,972)
Total current liabilities	(94,905)		(110,475)
Long-term debt	(185,021)		(138,974)
Other liabilities	(1,841,818)		(1,862,605)
Common stock	(52,017)		(52,017)
Additional paid-in capital	(502,625)		(464,888)
Retained earnings (deficit)	(448,832)		(441,860)
Total Steel Dynamics, Inc. equity	(1,003,474)		(958,765)
Total equity	(1,003,474)		(958,765)
Total liabilities and equity	$ (3,125,218)		$ (3,070,819)