Condensed Consolidating Information - Balance Sheets (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Condensed Consolidating Balance Sheets
Cash and equivalents	$ 972,561		$ 828,220
Short-term Investments	114,154		228,783
Accounts receivable, net	1,072,879		1,043,756
Inventories	1,802,759		1,859,168
Other current assets	61,248		72,730
Total current assets	4,023,601		4,032,657
Property, plant and equipment, net	2,947,243		2,945,767
Intangible assets, net	256,302		270,328
Goodwill	526,462		429,645
Other assets, including investments in subs	104,306		25,166
Total assets	7,857,914		7,703,563
Accounts payable	525,156		550,754
Accrued expenses	328,449		444,149
Current maturities of long-term debt	72,131		24,234
Total current liabilities	925,736		1,019,137
Long-term debt	2,355,917		2,352,489
Other liabilities	527,980		444,708
Total liabilities	3,809,633		3,816,334
Redeemable noncontrolling interests	139,930		111,240
Common stock	645		645
Treasury stock	(1,354,157)		(1,184,243)
Additional paid-in capital	1,167,505		1,160,048
Retained earnings (deficit)	4,250,419		3,958,320
Accumulated other comprehensive income (loss)	78		301
Total Steel Dynamics, Inc. equity	4,064,490		3,935,071
Noncontrolling interests	(156,139)		(159,082)
Total equity	3,908,351	$ 3,849,528	3,775,989	$ 3,588,519	$ 3,312,137	$ 3,195,068
Total liabilities and equity	7,857,914		7,703,563
Parent
Condensed Consolidating Balance Sheets
Cash and equivalents	914,541		809,763
Short-term Investments	109,154		198,783
Accounts receivable, net	324,548		340,439
Inventories	748,921		793,174
Other current assets	46,197		56,578
Total current assets	2,143,361		2,198,737
Property, plant and equipment, net	883,970		871,482
Other assets, including investments in subs	2,838,285		2,862,556
Total assets	5,865,616		5,932,775
Accounts payable	174,596		209,156
Accrued expenses	180,887		296,528
Current maturities of long-term debt	826		793
Total current liabilities	356,309		506,477
Long-term debt	2,329,916		2,327,798
Other liabilities	(885,161)		(836,571)
Total liabilities	1,801,064		1,997,704
Common stock	645		645
Treasury stock	(1,354,157)		(1,184,243)
Additional paid-in capital	1,167,505		1,160,048
Retained earnings (deficit)	4,250,419		3,958,320
Accumulated other comprehensive income (loss)	140		301
Total Steel Dynamics, Inc. equity	4,064,552		3,935,071
Total equity	4,064,552		3,935,071
Total liabilities and equity	5,865,616		5,932,775
Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	50,670		13,491
Short-term Investments	5,000		30,000
Accounts receivable, net	1,696,820		1,635,168
Inventories	977,754		1,038,702
Other current assets	18,698		18,627
Total current assets	2,748,942		2,735,988
Property, plant and equipment, net	1,902,632		1,918,198
Intangible assets, net	256,302		270,328
Goodwill	427,950		429,645
Other assets, including investments in subs	65,100		5,593
Total assets	5,400,926		5,359,752
Accounts payable	331,110		330,156
Accrued expenses	296,932		295,668
Current maturities of long-term debt	1,165		1,355
Total current liabilities	629,207		627,179
Long-term debt			381
Other liabilities	1,244,276		1,447,464
Total liabilities	1,873,483		2,075,024
Common stock	1,727,859		1,727,859
Additional paid-in capital	683,048		683,048
Retained earnings (deficit)	1,116,598		873,821
Accumulated other comprehensive income (loss)	(62)
Total Steel Dynamics, Inc. equity	3,527,443		3,284,728
Total equity	3,527,443		3,284,728
Total liabilities and equity	5,400,926		5,359,752
Combined Non-Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	7,350		4,966
Accounts receivable, net	70,968		26,655
Inventories	87,342		39,214
Other current assets	3,754		3,994
Total current assets	169,414		74,829
Property, plant and equipment, net	160,641		156,087
Goodwill	98,512
Other assets, including investments in subs	6,800		5,557
Total assets	435,367		236,473
Accounts payable	88,182		74,353
Accrued expenses	20,068		11,171
Current maturities of long-term debt	93,176		51,079
Total current liabilities	201,426		136,603
Long-term debt	170,842		166,226
Other liabilities	33,352		31,791
Total liabilities	405,620		334,620
Redeemable noncontrolling interests	139,930		111,240
Common stock	15,016		15,016
Additional paid-in capital	787,572		695,502
Retained earnings (deficit)	(756,632)		(760,823)
Total Steel Dynamics, Inc. equity	45,956		(50,305)
Noncontrolling interests	(156,139)		(159,082)
Total equity	(110,183)		(209,387)
Total liabilities and equity	435,367		236,473
Consolidating Adjustments
Condensed Consolidating Balance Sheets
Accounts receivable, net	(1,019,457)		(958,506)
Inventories	(11,258)		(11,922)
Other current assets	(7,401)		(6,469)
Total current assets	(1,038,116)		(976,897)
Other assets, including investments in subs	(2,805,879)		(2,848,540)
Total assets	(3,843,995)		(3,825,437)
Accounts payable	(68,732)		(62,911)
Accrued expenses	(169,438)		(159,218)
Current maturities of long-term debt	(23,036)		(28,993)
Total current liabilities	(261,206)		(251,122)
Long-term debt	(144,841)		(141,916)
Other liabilities	135,513		(197,976)
Total liabilities	(270,534)		(591,014)
Common stock	(1,742,875)		(1,742,875)
Additional paid-in capital	(1,470,620)		(1,378,550)
Retained earnings (deficit)	(359,966)		(112,998)
Total Steel Dynamics, Inc. equity	(3,573,461)		(3,234,423)
Total equity	(3,573,461)		(3,234,423)
Total liabilities and equity	$ (3,843,995)		$ (3,825,437)