Condensed Consolidating Information - Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Condensed Consolidating Balance Sheets
Cash and equivalents	$ 1,146,007			$ 828,220
Short-term Investments	69,529			228,783
Accounts receivable, net	986,946			1,043,756
Inventories	1,767,020			1,859,168
Other current assets	50,818			72,730
Total current assets	4,020,320			4,032,657
Property, plant and equipment, net	3,031,731			2,945,767
Intangible assets, net	249,598			270,328
Goodwill	540,913			429,645
Other assets, including investments in subs	100,685			25,166
Total assets	7,943,247			7,703,563
Accounts payable	528,629			550,754
Accrued expenses	394,496			444,149
Current maturities of long-term debt	82,150			24,234
Total current liabilities	1,005,275			1,019,137
Long-term debt	2,355,243			2,352,489
Other liabilities	538,374			444,708
Total liabilities	3,898,892			3,816,334
Redeemable noncontrolling interests	143,614			111,240
Common stock	645			645
Treasury stock	(1,469,078)			(1,184,243)
Additional paid-in capital	1,175,512			1,160,048
Retained earnings (deficit)	4,349,523			3,958,320
Accumulated other comprehensive income (loss)	39			301
Total Steel Dynamics, Inc. equity	4,056,641			3,935,071
Noncontrolling interests	(155,900)			(159,082)
Total equity	3,900,741	$ 3,908,351	$ 3,849,528	3,775,989	$ 3,874,837	$ 3,588,519	$ 3,312,137	$ 3,195,068
Total liabilities and equity	7,943,247			7,703,563
Parent
Condensed Consolidating Balance Sheets
Cash and equivalents	1,070,215			809,763
Short-term Investments	69,529			198,783
Accounts receivable, net	303,259			340,439
Inventories	725,792			793,174
Other current assets	29,754			56,578
Total current assets	2,198,549			2,198,737
Property, plant and equipment, net	966,715			871,482
Other assets, including investments in subs	2,723,786			2,862,556
Total assets	5,889,050			5,932,775
Accounts payable	177,308			209,156
Accrued expenses	228,600			296,528
Current maturities of long-term debt	843			793
Total current liabilities	406,751			506,477
Long-term debt	2,330,968			2,327,798
Other liabilities	(905,310)			(836,571)
Total liabilities	1,832,409			1,997,704
Common stock	645			645
Treasury stock	(1,469,078)			(1,184,243)
Additional paid-in capital	1,175,512			1,160,048
Retained earnings (deficit)	4,349,523			3,958,320
Accumulated other comprehensive income (loss)	39			301
Total Steel Dynamics, Inc. equity	4,056,641			3,935,071
Total equity	4,056,641			3,935,071
Total liabilities and equity	5,889,050			5,932,775
Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	64,398			13,491
Short-term Investments				30,000
Accounts receivable, net	1,675,521			1,635,168
Inventories	960,506			1,038,702
Other current assets	17,464			18,627
Total current assets	2,717,889			2,735,988
Property, plant and equipment, net	1,902,414			1,918,198
Intangible assets, net	249,598			270,328
Goodwill	427,103			429,645
Other assets, including investments in subs	61,910			5,593
Total assets	5,358,914			5,359,752
Accounts payable	332,299			330,156
Accrued expenses	316,684			295,668
Current maturities of long-term debt	517			1,355
Total current liabilities	649,500			627,179
Long-term debt				381
Other liabilities	1,087,758			1,447,464
Total liabilities	1,737,258			2,075,024
Common stock	1,727,859			1,727,859
Additional paid-in capital	683,048			683,048
Retained earnings (deficit)	1,210,749			873,821
Total Steel Dynamics, Inc. equity	3,621,656			3,284,728
Total equity	3,621,656			3,284,728
Total liabilities and equity	5,358,914			5,359,752
Combined Non-Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	11,394			4,966
Accounts receivable, net	63,962			26,655
Inventories	88,871			39,214
Other current assets	4,548			3,994
Total current assets	168,775			74,829
Property, plant and equipment, net	162,602			156,087
Goodwill	113,810
Other assets, including investments in subs	6,420			5,557
Total assets	451,607			236,473
Accounts payable	99,162			74,353
Accrued expenses	16,694			11,171
Current maturities of long-term debt	103,551			51,079
Total current liabilities	219,407			136,603
Long-term debt	147,310			166,226
Other liabilities	34,290			31,791
Total liabilities	401,007			334,620
Redeemable noncontrolling interests	143,614			111,240
Common stock	15,016			15,016
Additional paid-in capital	804,625			695,502
Retained earnings (deficit)	(756,755)			(760,823)
Total Steel Dynamics, Inc. equity	62,886			(50,305)
Noncontrolling interests	(155,900)			(159,082)
Total equity	(93,014)			(209,387)
Total liabilities and equity	451,607			236,473
Consolidating Adjustments
Condensed Consolidating Balance Sheets
Accounts receivable, net	(1,055,796)			(958,506)
Inventories	(8,149)			(11,922)
Other current assets	(948)			(6,469)
Total current assets	(1,064,893)			(976,897)
Other assets, including investments in subs	(2,691,431)			(2,848,540)
Total assets	(3,756,324)			(3,825,437)
Accounts payable	(80,140)			(62,911)
Accrued expenses	(167,482)			(159,218)
Current maturities of long-term debt	(22,761)			(28,993)
Total current liabilities	(270,383)			(251,122)
Long-term debt	(123,035)			(141,916)
Other liabilities	321,636			(197,976)
Total liabilities	(71,782)			(591,014)
Common stock	(1,742,875)			(1,742,875)
Additional paid-in capital	(1,487,673)			(1,378,550)
Retained earnings (deficit)	(453,994)			(112,998)
Total Steel Dynamics, Inc. equity	(3,684,542)			(3,234,423)
Total equity	(3,684,542)			(3,234,423)
Total liabilities and equity	$ (3,756,324)			$ (3,825,437)