Condensed Consolidating Information - Balance Sheets (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Condensed Consolidating Balance Sheets
Cash and equivalents	$ 985,824	$ 1,028,649
Short-term Investments	40,000
Accounts receivable, net	987,655	868,837
Inventories	1,600,058	1,519,347
Other current assets	38,705	91,509
Total current assets	3,652,242	3,508,342
Property, plant and equipment, net	2,657,937	2,675,904
Intangible assets, net	249,983	256,909
Goodwill	386,045	386,893
Other assets, including investments in subs	26,606	27,684
Total assets	6,972,813	6,855,732
Accounts payable	562,618	489,448
Accrued expenses	288,476	350,276
Current maturities of long-term debt	9,646	28,795
Total current liabilities	860,740	868,519
Long-term debt	2,353,703	2,353,145
Other liabilities	334,993	327,760
Total liabilities	3,549,436	3,549,424
Redeemable noncontrolling interests	111,240	111,240
Common stock	644	644
Treasury stock	(730,700)	(665,297)
Additional paid-in capital	1,142,871	1,141,534
Retained earnings (deficit)	3,057,904	2,874,693
Total Steel Dynamics, Inc. equity	3,470,719	3,351,574
Noncontrolling interests	(158,582)	(156,506)
Total equity	3,312,137	3,195,068
Total liabilities and equity	6,972,813	6,855,732
Parent
Condensed Consolidating Balance Sheets
Cash and equivalents	969,240	1,001,405
Short-term Investments	40,000
Accounts receivable, net	349,276	274,968
Inventories	743,939	685,103
Other current assets	23,717	73,748
Total current assets	2,126,172	2,035,224
Property, plant and equipment, net	854,685	859,419
Other assets, including investments in subs	2,486,756	2,512,594
Total assets	5,467,613	5,407,237
Accounts payable	185,459	168,282
Accrued expenses	185,020	222,023
Current maturities of long-term debt	746	731
Total current liabilities	371,225	391,036
Long-term debt	2,327,621	2,326,466
Other liabilities	(701,952)	(661,839)
Total liabilities	1,996,894	2,055,663
Common stock	644	644
Treasury stock	(730,700)	(665,297)
Additional paid-in capital	1,142,871	1,141,534
Retained earnings (deficit)	3,057,904	2,874,693
Total Steel Dynamics, Inc. equity	3,470,719	3,351,574
Total equity	3,470,719	3,351,574
Total liabilities and equity	5,467,613	5,407,237
Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	9,294	20,441
Accounts receivable, net	1,475,249	1,426,036
Inventories	804,504	752,151
Other current assets	14,814	16,005
Total current assets	2,303,861	2,214,633
Property, plant and equipment, net	1,605,485	1,618,438
Intangible assets, net	219,374	225,503
Goodwill	378,221	379,069
Other assets, including investments in subs	6,081	6,622
Total assets	4,513,022	4,444,265
Accounts payable	363,178	316,676
Accrued expenses	232,542	254,196
Total current liabilities	595,720	570,872
Other liabilities	775,007	869,196
Total liabilities	1,370,727	1,440,068
Common stock	1,727,859	1,727,859
Additional paid-in capital	128,076	128,076
Retained earnings (deficit)	1,286,360	1,148,262
Total Steel Dynamics, Inc. equity	3,142,295	3,004,197
Total equity	3,142,295	3,004,197
Total liabilities and equity	4,513,022	4,444,265
Combined Non-Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	7,290	6,803
Accounts receivable, net	33,922	37,387
Inventories	59,192	91,890
Other current assets	5,095	5,962
Total current assets	105,499	142,042
Property, plant and equipment, net	197,767	198,047
Intangible assets, net	30,609	31,406
Goodwill	7,824	7,824
Other assets, including investments in subs	5,588	5,505
Total assets	347,287	384,824
Accounts payable	87,276	101,948
Accrued expenses	11,103	10,243
Current maturities of long-term debt	36,761	56,454
Total current liabilities	135,140	168,645
Long-term debt	167,787	169,799
Other liabilities	22,547	24,868
Total liabilities	325,474	363,312
Redeemable noncontrolling interests	111,240	111,240
Common stock	15,016	14,908
Additional paid-in capital	803,204	797,196
Retained earnings (deficit)	(749,065)	(745,326)
Total Steel Dynamics, Inc. equity	69,155	66,778
Noncontrolling interests	(158,582)	(156,506)
Total equity	(89,427)	(89,728)
Total liabilities and equity	347,287	384,824
Consolidating Adjustments
Condensed Consolidating Balance Sheets
Accounts receivable, net	(870,792)	(869,554)
Inventories	(7,577)	(9,797)
Other current assets	(4,921)	(4,206)
Total current assets	(883,290)	(883,557)
Other assets, including investments in subs	(2,471,819)	(2,497,037)
Total assets	(3,355,109)	(3,380,594)
Accounts payable	(73,295)	(97,458)
Accrued expenses	(140,189)	(136,186)
Current maturities of long-term debt	(27,861)	(28,390)
Total current liabilities	(241,345)	(262,034)
Long-term debt	(141,705)	(143,120)
Other liabilities	239,391	95,535
Total liabilities	(143,659)	(309,619)
Common stock	(1,742,875)	(1,742,767)
Additional paid-in capital	(931,280)	(925,272)
Retained earnings (deficit)	(537,295)	(402,936)
Total Steel Dynamics, Inc. equity	(3,211,450)	(3,070,975)
Total equity	(3,211,450)	(3,070,975)
Total liabilities and equity	$ (3,355,109)	$ (3,380,594)