Condensed Consolidating Information - Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Consolidating Balance Sheets
Cash and equivalents	$ 1,028,649	$ 841,483	$ 727,032	$ 361,363
Accounts receivable, net	868,837	729,784
Inventories	1,519,347	1,275,211
Other current assets	91,509	83,197
Total current assets	3,508,342	2,929,675
Property, plant and equipment, net	2,675,904	2,787,215
Intangible assets, net	256,909	283,977
Goodwill	386,893	393,351
Other assets, including investments in subs	27,684	29,514
Total assets	6,855,732	6,423,732
Accounts payable	489,448	395,196
Accrued expenses	350,276	313,987
Current maturities of long-term debt	28,795	3,632
Total current liabilities	868,519	712,815
Long-term debt	2,353,145	2,353,194
Other liabilities	327,760	469,024
Total liabilities	3,549,424	3,535,033
Redeemable noncontrolling interests	111,240	111,240
Common stock	644	641
Treasury stock	(665,297)	(416,829)
Additional paid-in capital	1,141,534	1,132,749
Retained earnings (deficit)	2,874,693	2,210,459
Total Steel Dynamics, Inc. equity	3,351,574	2,927,020
Noncontrolling interests	(156,506)	(149,561)
Total equity	3,195,068	2,777,459	2,545,111	2,795,527
Total liabilities and equity	6,855,732	6,423,732
Parent
Condensed Consolidating Balance Sheets
Cash and equivalents	1,001,405	766,685	636,877	265,313
Accounts receivable, net	274,968	229,148
Inventories	685,103	587,319
Other current assets	73,748	45,049
Total current assets	2,035,224	1,628,201
Property, plant and equipment, net	859,419	899,370
Other assets, including investments in subs	2,512,594	2,769,884
Total assets	5,407,237	5,297,455
Accounts payable	168,282	141,089
Accrued expenses	222,023	198,085
Current maturities of long-term debt	731	674
Total current liabilities	391,036	339,848
Long-term debt	2,326,466	2,324,298
Other liabilities	(661,839)	(293,711)
Total liabilities	2,055,663	2,370,435
Common stock	644	641
Treasury stock	(665,297)	(416,829)
Additional paid-in capital	1,141,534	1,132,749
Retained earnings (deficit)	2,874,693	2,210,459
Total Steel Dynamics, Inc. equity	3,351,574	2,927,020
Total equity	3,351,574	2,927,020
Total liabilities and equity	5,407,237	5,297,455
Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	20,441	54,677	81,976	81,690
Accounts receivable, net	1,426,036	1,257,245
Inventories	752,151	639,148
Other current assets	16,005	36,062
Total current assets	2,214,633	1,987,132
Property, plant and equipment, net	1,618,438	1,679,751
Intangible assets, net	225,503	251,919
Goodwill	379,069	385,527
Other assets, including investments in subs	6,622	7,335
Total assets	4,444,265	4,311,664
Accounts payable	316,676	265,764
Accrued expenses	254,196	220,917
Current maturities of long-term debt		700
Total current liabilities	570,872	487,381
Other liabilities	869,196	1,219,444
Total liabilities	1,440,068	1,706,825
Common stock	1,727,859	1,727,859
Additional paid-in capital	128,076	128,076
Retained earnings (deficit)	1,148,262	748,904
Total Steel Dynamics, Inc. equity	3,004,197	2,604,839
Total equity	3,004,197	2,604,839
Total liabilities and equity	4,444,265	4,311,664
Combined Non-Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	6,803	20,121	$ 8,179	$ 14,360
Accounts receivable, net	37,387	23,689
Inventories	91,890	58,696
Other current assets	5,962	4,447
Total current assets	142,042	106,953
Property, plant and equipment, net	198,047	208,094
Intangible assets, net	31,406	32,058
Goodwill	7,824	7,824
Other assets, including investments in subs	5,505	5,832
Total assets	384,824	360,761
Accounts payable	101,948	89,659
Accrued expenses	10,243	8,793
Current maturities of long-term debt	56,454	29,347
Total current liabilities	168,645	127,799
Long-term debt	169,799	168,566
Other liabilities	24,868	42,482
Total liabilities	363,312	338,847
Redeemable noncontrolling interests	111,240	111,240
Common stock	14,908	14,908
Additional paid-in capital	797,196	779,678
Retained earnings (deficit)	(745,326)	(734,351)
Total Steel Dynamics, Inc. equity	66,778	60,235
Noncontrolling interests	(156,506)	(149,561)
Total equity	(89,728)	(89,326)
Total liabilities and equity	384,824	360,761
Consolidating Adjustments
Condensed Consolidating Balance Sheets
Accounts receivable, net	(869,554)	(780,298)
Inventories	(9,797)	(9,952)
Other current assets	(4,206)	(2,361)
Total current assets	(883,557)	(792,611)
Other assets, including investments in subs	(2,497,037)	(2,753,537)
Total assets	(3,380,594)	(3,546,148)
Accounts payable	(97,458)	(101,316)
Accrued expenses	(136,186)	(113,808)
Current maturities of long-term debt	(28,390)	(27,089)
Total current liabilities	(262,034)	(242,213)
Long-term debt	(143,120)	(139,670)
Other liabilities	95,535	(499,191)
Total liabilities	(309,619)	(881,074)
Common stock	(1,742,767)	(1,742,767)
Additional paid-in capital	(925,272)	(907,754)
Retained earnings (deficit)	(402,936)	(14,553)
Total Steel Dynamics, Inc. equity	(3,070,975)	(2,665,074)
Total equity	(3,070,975)	(2,665,074)
Total liabilities and equity	$ (3,380,594)	$ (3,546,148)