Condensed Consolidating Information - Balance Sheets (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Condensed Consolidating Balance Sheets
Cash and equivalents	$ 791,444	$ 828,220
Short-term Investments	173,723	228,783
Accounts receivable, net	1,143,395	1,043,756
Inventories	1,867,700	1,859,168
Other current assets	52,628	72,730
Total current assets	4,028,890	4,032,657
Property, plant and equipment, net	2,936,893	2,945,767
Intangible assets, net	263,315	270,328
Goodwill	530,716	429,645
Other assets, including investments in subs	97,419	25,166
Total assets	7,857,233	7,703,563
Accounts payable	591,467	550,754
Accrued expenses	330,665	444,149
Current maturities of long-term debt	80,958	24,234
Total current liabilities	1,003,090	1,019,137
Long-term debt	2,354,427	2,352,489
Other liabilities	510,258	444,708
Total liabilities	3,867,775	3,816,334
Redeemable noncontrolling interests	139,930	111,240
Common stock	645	645
Treasury stock	(1,261,837)	(1,184,243)
Additional paid-in capital	1,160,139	1,160,048
Retained earnings (deficit)	4,109,034	3,958,320
Accumulated other comprehensive income (loss)	130	301
Total Steel Dynamics, Inc. equity	4,008,111	3,935,071
Noncontrolling interests	(158,583)	(159,082)
Total equity	3,849,528	3,775,989	$ 3,312,137	$ 3,195,068
Total liabilities and equity	7,857,233	7,703,563
Parent
Condensed Consolidating Balance Sheets
Cash and equivalents	755,014	809,763
Short-term Investments	153,723	198,783
Accounts receivable, net	344,209	340,439
Inventories	786,300	793,174
Other current assets	38,977	56,578
Total current assets	2,078,223	2,198,737
Property, plant and equipment, net	870,626	871,482
Other assets, including investments in subs	2,934,875	2,862,556
Total assets	5,883,724	5,932,775
Accounts payable	197,481	209,156
Accrued expenses	200,512	296,528
Current maturities of long-term debt	809	793
Total current liabilities	398,802	506,477
Long-term debt	2,328,859	2,327,798
Other liabilities	(852,048)	(836,571)
Total liabilities	1,875,613	1,997,704
Common stock	645	645
Treasury stock	(1,261,837)	(1,184,243)
Additional paid-in capital	1,160,139	1,160,048
Retained earnings (deficit)	4,109,034	3,958,320
Accumulated other comprehensive income (loss)	130	301
Total Steel Dynamics, Inc. equity	4,008,111	3,935,071
Total equity	4,008,111	3,935,071
Total liabilities and equity	5,883,724	5,932,775
Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	26,067	13,491
Short-term Investments	20,000	30,000
Accounts receivable, net	1,727,028	1,635,168
Inventories	1,010,636	1,038,702
Other current assets	17,243	18,627
Total current assets	2,800,974	2,735,988
Property, plant and equipment, net	1,905,151	1,918,198
Intangible assets, net	263,315	270,328
Goodwill	428,798	429,645
Other assets, including investments in subs	59,075	5,593
Total assets	5,457,313	5,359,752
Accounts payable	376,400	330,156
Accrued expenses	275,807	295,668
Current maturities of long-term debt	1,116	1,355
Total current liabilities	653,323	627,179
Long-term debt		381
Other liabilities	1,398,528	1,447,464
Total liabilities	2,051,851	2,075,024
Common stock	1,727,859	1,727,859
Additional paid-in capital	683,048	683,048
Retained earnings (deficit)	994,555	873,821
Total Steel Dynamics, Inc. equity	3,405,462	3,284,728
Total equity	3,405,462	3,284,728
Total liabilities and equity	5,457,313	5,359,752
Combined Non-Guarantors
Condensed Consolidating Balance Sheets
Cash and equivalents	10,363	4,966
Accounts receivable, net	71,953	26,655
Inventories	84,420	39,214
Other current assets	3,331	3,994
Total current assets	170,067	74,829
Property, plant and equipment, net	161,116	156,087
Goodwill	101,918
Other assets, including investments in subs	5,850	5,557
Total assets	438,951	236,473
Accounts payable	94,697	74,353
Accrued expenses	17,763	11,171
Current maturities of long-term debt	104,574	51,079
Total current liabilities	217,034	136,603
Long-term debt	166,353	166,226
Other liabilities	33,483	31,791
Total liabilities	416,870	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)	(761,854)	(760,823)
Total Steel Dynamics, Inc. equity	40,734	(50,305)
Noncontrolling interests	(158,583)	(159,082)
Total equity	(117,849)	(209,387)
Total liabilities and equity	438,951	236,473
Consolidating Adjustments
Condensed Consolidating Balance Sheets
Accounts receivable, net	(999,795)	(958,506)
Inventories	(13,656)	(11,922)
Other current assets	(6,923)	(6,469)
Total current assets	(1,020,374)	(976,897)
Other assets, including investments in subs	(2,902,381)	(2,848,540)
Total assets	(3,922,755)	(3,825,437)
Accounts payable	(77,111)	(62,911)
Accrued expenses	(163,417)	(159,218)
Current maturities of long-term debt	(25,541)	(28,993)
Total current liabilities	(266,069)	(251,122)
Long-term debt	(140,785)	(141,916)
Other liabilities	(69,705)	(197,976)
Total liabilities	(476,559)	(591,014)
Common stock	(1,742,875)	(1,742,875)
Additional paid-in capital	(1,470,620)	(1,378,550)
Retained earnings (deficit)	(232,701)	(112,998)
Total Steel Dynamics, Inc. equity	(3,446,196)	(3,234,423)
Total equity	(3,446,196)	(3,234,423)
Total liabilities and equity	$ (3,922,755)	$ (3,825,437)