Condensed Consolidating Information - Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Consolidating Balance Sheets
Cash and equivalents	$ 841,483	$ 727,032	$ 361,363	$ 395,156
Accounts receivable, net	729,784	613,605
Inventories	1,275,211	1,149,390
Other current assets	83,197	47,914
Total current assets	2,929,675	2,537,941
Property, plant and equipment, net	2,787,215	2,951,210
Intangible assets, net	283,977	278,960
Goodwill	393,351	397,470
Other assets, including investments in subs	29,514	36,501
Total assets	6,423,732	6,202,082
Accounts payable	395,196	283,355
Accrued expenses	313,987	235,255
Current maturities of long-term debt	3,632	16,680
Total current liabilities	712,815	535,290
Long-term debt	2,353,194	2,577,976
Other liabilities	469,024	417,365
Total liabilities	3,535,033	3,530,631
Redeemable noncontrolling interests	111,240	126,340
Common stock	641	638
Treasury stock	(416,829)	(396,455)
Additional paid-in capital	1,132,749	1,110,253
Retained earnings (deficit)	2,210,459	1,965,291
Total Steel Dynamics, Inc. equity	2,927,020	2,679,727
Noncontrolling interests	(149,561)	(134,616)
Total equity	2,777,459	2,545,111	2,795,527	2,495,855
Total liabilities and equity	6,423,732	6,202,082
Parent
Condensed Consolidating Balance Sheets
Cash and equivalents	766,685	636,877	265,313	320,866
Accounts receivable, net	229,148	200,094
Inventories	587,319	539,963
Other current assets	45,049	21,654
Total current assets	1,628,201	1,398,588
Property, plant and equipment, net	899,370	958,212
Other assets, including investments in subs	2,769,884	2,941,710
Total assets	5,297,455	5,298,510
Accounts payable	141,089	100,751
Accrued expenses	198,085	141,552
Current maturities of long-term debt	674	13,122
Total current liabilities	339,848	255,425
Long-term debt	2,324,298	2,546,606
Other liabilities	(293,711)	(183,248)
Total liabilities	2,370,435	2,618,783
Common stock	641	638
Treasury stock	(416,829)	(396,455)
Additional paid-in capital	1,132,749	1,110,253
Retained earnings (deficit)	2,210,459	1,965,291
Total Steel Dynamics, Inc. equity	2,927,020	2,679,727
Total equity	2,927,020	2,679,727
Total liabilities and equity	5,297,455	5,298,510
Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	54,677	81,976	81,690	61,148
Accounts receivable, net	1,257,245	1,056,285
Inventories	639,148	573,924
Other current assets	36,062	25,415
Total current assets	1,987,132	1,737,600
Property, plant and equipment, net	1,679,751	1,703,932
Intangible assets, net	251,919	278,960
Goodwill	385,527	397,470
Other assets, including investments in subs	7,335	10,040
Total assets	4,311,664	4,128,002
Accounts payable	265,764	183,344
Accrued expenses	220,917	185,873
Current maturities of long-term debt	700	700
Total current liabilities	487,381	369,917
Long-term debt		361
Other liabilities	1,219,444	1,342,541
Total liabilities	1,706,825	1,712,819
Common stock	1,727,859	1,727,859
Additional paid-in capital	128,076	117,737
Retained earnings (deficit)	748,904	569,587
Total Steel Dynamics, Inc. equity	2,604,839	2,415,183
Total equity	2,604,839	2,415,183
Total liabilities and equity	4,311,664	4,128,002
Combined Non-Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	20,121	8,179	$ 14,360	$ 13,142
Accounts receivable, net	23,689	29,775
Inventories	58,696	35,004
Other current assets	4,447	1,676
Total current assets	106,953	74,634
Property, plant and equipment, net	208,094	291,077
Intangible assets, net	32,058
Goodwill	7,824
Other assets, including investments in subs	5,832	6,137
Total assets	360,761	371,848
Accounts payable	89,659	68,948
Accrued expenses	8,793	4,779
Current maturities of long-term debt	29,347	24,975
Total current liabilities	127,799	98,702
Long-term debt	168,566	177,897
Other liabilities	42,482	63,020
Total liabilities	338,847	339,619
Redeemable noncontrolling interests	111,240	126,340
Common stock	14,908	18,120
Additional paid-in capital	779,678	646,787
Retained earnings (deficit)	(734,351)	(624,402)
Total Steel Dynamics, Inc. equity	60,235	40,505
Noncontrolling interests	(149,561)	(134,616)
Total equity	(89,326)	(94,111)
Total liabilities and equity	360,761	371,848
Consolidating Adjustments
Condensed Consolidating Balance Sheets
Accounts receivable, net	(780,298)	(672,549)
Inventories	(9,952)	499
Other current assets	(2,361)	(831)
Total current assets	(792,611)	(672,881)
Property, plant and equipment, net		(2,011)
Other assets, including investments in subs	(2,753,537)	(2,921,386)
Total assets	(3,546,148)	(3,596,278)
Accounts payable	(101,316)	(69,688)
Accrued expenses	(113,808)	(96,949)
Current maturities of long-term debt	(27,089)	(22,117)
Total current liabilities	(242,213)	(188,754)
Long-term debt	(139,670)	(146,888)
Other liabilities	(499,191)	(804,948)
Total liabilities	(881,074)	(1,140,590)
Common stock	(1,742,767)	(1,745,979)
Additional paid-in capital	(907,754)	(764,524)
Retained earnings (deficit)	(14,553)	54,815
Total Steel Dynamics, Inc. equity	(2,665,074)	(2,455,688)
Total equity	(2,665,074)	(2,455,688)
Total liabilities and equity	$ (3,546,148)	$ (3,596,278)