Condensed Consolidating Information - Balance Sheets (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Condensed Consolidating Balance Sheets
Cash and equivalents	$ 966,826	$ 841,483	$ 976,951	$ 727,032
Accounts receivable, net	883,147	729,784
Inventories	1,361,550	1,275,211
Other current assets	33,442	83,197
Total current assets	3,244,965	2,929,675
Property, plant and equipment, net	2,760,544	2,787,215
Intangible assets, net	276,553	283,977
Goodwill	391,740	393,351
Other assets, including investments in subs	30,233	29,514
Total assets	6,704,035	6,423,732
Accounts payable	530,958	395,196
Accrued expenses	347,524	313,987
Current maturities of long-term debt	2,965	3,632
Total current liabilities	881,447	712,815
Long-term debt	2,353,744	2,353,194
Other liabilities	474,137	469,024
Total liabilities	3,709,328	3,535,033
Redeemable noncontrolling interests	111,240	111,240
Common stock	641	641
Treasury stock	(475,072)	(416,829)
Additional paid-in capital	1,135,892	1,132,749
Retained earnings (deficit)	2,373,718	2,210,459
Total Steel Dynamics, Inc. equity	3,035,179	2,927,020
Noncontrolling interests	(151,712)	(149,561)
Total equity	2,883,467	2,777,459
Total liabilities and equity	6,704,035	6,423,732
Parent
Condensed Consolidating Balance Sheets
Cash and equivalents	936,047	766,685	887,690	636,877
Accounts receivable, net	277,244	229,148
Inventories	624,594	587,319
Other current assets	22,277	45,049
Total current assets	1,860,162	1,628,201
Property, plant and equipment, net	889,697	899,370
Other assets, including investments in subs	2,696,497	2,769,884
Total assets	5,446,356	5,297,455
Accounts payable	195,273	141,089
Accrued expenses	248,907	198,085
Current maturities of long-term debt	688	674
Total current liabilities	444,868	339,848
Long-term debt	2,325,394	2,324,298
Other liabilities	(359,085)	(293,711)
Total liabilities	2,411,177	2,370,435
Common stock	641	641
Treasury stock	(475,072)	(416,829)
Additional paid-in capital	1,135,892	1,132,749
Retained earnings (deficit)	2,373,718	2,210,459
Total Steel Dynamics, Inc. equity	3,035,179	2,927,020
Total equity	3,035,179	2,927,020
Total liabilities and equity	5,446,356	5,297,455
Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	10,986	54,677	75,082	81,976
Accounts receivable, net	1,372,691	1,257,245
Inventories	694,956	639,148
Other current assets	9,026	36,062
Total current assets	2,087,659	1,987,132
Property, plant and equipment, net	1,666,091	1,679,751
Intangible assets, net	245,158	251,919
Goodwill	383,916	385,527
Other assets, including investments in subs	7,678	7,335
Total assets	4,390,502	4,311,664
Accounts payable	341,361	265,764
Accrued expenses	209,587	220,917
Current maturities of long-term debt		700
Total current liabilities	550,948	487,381
Other liabilities	1,111,191	1,219,444
Total liabilities	1,662,139	1,706,825
Common stock	1,727,859	1,727,859
Additional paid-in capital	128,076	128,076
Retained earnings (deficit)	872,428	748,904
Total Steel Dynamics, Inc. equity	2,728,363	2,604,839
Total equity	2,728,363	2,604,839
Total liabilities and equity	4,390,502	4,311,664
Combined Non-Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	19,793	20,121	$ 14,179	$ 8,179
Accounts receivable, net	35,115	23,689
Inventories	52,100	58,696
Other current assets	5,101	4,447
Total current assets	112,109	106,953
Property, plant and equipment, net	204,756	208,094
Intangible assets, net	31,395	32,058
Goodwill	7,824	7,824
Other assets, including investments in subs	5,560	5,832
Total assets	361,644	360,761
Accounts payable	95,142	89,659
Accrued expenses	7,249	8,793
Current maturities of long-term debt	32,345	29,347
Total current liabilities	134,736	127,799
Long-term debt	171,392	168,566
Other liabilities	38,108	42,482
Total liabilities	344,236	338,847
Redeemable noncontrolling interests	111,240	111,240
Common stock	14,908	14,908
Additional paid-in capital	779,678	779,678
Retained earnings (deficit)	(736,706)	(734,351)
Total Steel Dynamics, Inc. equity	57,880	60,235
Noncontrolling interests	(151,712)	(149,561)
Total equity	(93,832)	(89,326)
Total liabilities and equity	361,644	360,761
Consolidating Adjustments
Condensed Consolidating Balance Sheets
Accounts receivable, net	(801,903)	(780,298)
Inventories	(10,100)	(9,952)
Other current assets	(2,962)	(2,361)
Total current assets	(814,965)	(792,611)
Other assets, including investments in subs	(2,679,502)	(2,753,537)
Total assets	(3,494,467)	(3,546,148)
Accounts payable	(100,818)	(101,316)
Accrued expenses	(118,219)	(113,808)
Current maturities of long-term debt	(30,068)	(27,089)
Total current liabilities	(249,105)	(242,213)
Long-term debt	(143,042)	(139,670)
Other liabilities	(316,077)	(499,191)
Total liabilities	(708,224)	(881,074)
Common stock	(1,742,767)	(1,742,767)
Additional paid-in capital	(907,754)	(907,754)
Retained earnings (deficit)	(135,722)	(14,553)
Total Steel Dynamics, Inc. equity	(2,786,243)	(2,665,074)
Total equity	(2,786,243)	(2,665,074)
Total liabilities and equity	$ (3,494,467)	$ (3,546,148)