Segment Information - Results (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information
Net sales	$ 2,601,245	$ 2,330,832	$ 2,094,305	$ 2,575,100	$ 2,350,196	$ 2,526,845	$ 2,770,515	$ 2,817,435	$ 9,601,482	$ 10,464,991	$ 11,821,839
Operating income (loss)	258,750	$ 155,856	$ 158,850	$ 273,686	181,961	$ 228,045	$ 285,032	$ 291,842	847,142	986,880	1,722,409
Income (loss) before income taxes									705,478	875,337	1,619,774
Depreciation and amortization									325,789	321,082	317,198
Capital expenditures									1,198,055	451,945	239,390
Assets	9,265,562				8,275,765				9,265,562	8,275,765
U.S.
Segment Reporting Information
Net sales									9,089,194	9,927,874
U.S. | Other segments
Segment Reporting Information
Net sales											11,120,013
Non-U.S.
Segment Reporting Information
Net sales									512,288	537,117	701,826
Operating Segment | Steel Operations
Segment Reporting Information
Net sales									7,455,637	8,234,179	9,263,152
Operating income (loss)									889,480	1,030,554	1,839,852
Income (loss) before income taxes									833,035	963,514	1,770,888
Depreciation and amortization									251,590	251,568	248,765
Capital expenditures									1,132,298	385,058	189,208
Assets	6,288,362				5,171,026				6,288,362	5,171,026
Operating Segment | Steel Operations | Other segments
Segment Reporting Information
Net sales									318,533	329,627	342,433
Operating Segment | Steel Operations | U.S.
Segment Reporting Information
Net sales									6,873,209	7,606,471
Operating Segment | Steel Operations | U.S. | Other segments
Segment Reporting Information
Net sales											8,476,104
Operating Segment | Steel Operations | Non-U.S.
Segment Reporting Information
Net sales									263,895	298,081	444,615
Operating Segment | Metals Recycling Operations
Segment Reporting Information
Net sales									2,403,140	2,494,014	3,202,214
Operating income (loss)									32,991	16,308	75,891
Income (loss) before income taxes									27,753	11,432	69,861
Depreciation and amortization									50,099	46,538	46,015
Capital expenditures									32,875	46,959	35,518
Assets	1,116,051				940,250				1,116,051	940,250
Operating Segment | Metals Recycling Operations | Other segments
Segment Reporting Information
Net sales									1,335,216	1,296,007	1,649,614
Operating Segment | Metals Recycling Operations | U.S.
Segment Reporting Information
Net sales									820,262	972,200
Operating Segment | Metals Recycling Operations | U.S. | Other segments
Segment Reporting Information
Net sales											1,295,514
Operating Segment | Metals Recycling Operations | Non-U.S.
Segment Reporting Information
Net sales									247,662	225,807	257,086
Operating Segment | Steel Fabrication Operations
Segment Reporting Information
Net sales									906,364	963,259	921,951
Operating income (loss)									120,575	119,099	61,901
Income (loss) before income taxes									116,625	114,359	56,236
Depreciation and amortization									10,819	11,768	11,553
Capital expenditures									15,234	12,446	8,303
Assets	376,658				386,485				376,658	386,485
Operating Segment | Steel Fabrication Operations | Other segments
Segment Reporting Information
Net sales									10,663	1,105	803
Operating Segment | Steel Fabrication Operations | U.S.
Segment Reporting Information
Net sales									895,227	960,657
Operating Segment | Steel Fabrication Operations | U.S. | Other segments
Segment Reporting Information
Net sales											921,023
Operating Segment | Steel Fabrication Operations | Non-U.S.
Segment Reporting Information
Net sales									474	1,497	125
Operating Segment | Other
Segment Reporting Information
Net sales									501,187	400,747	429,060
Operating income (loss)									(188,525)	(186,159)	(253,195)
Income (loss) before income taxes									(263,470)	(220,188)	(275,137)
Depreciation and amortization									13,281	11,208	10,865
Capital expenditures									17,648	7,482	6,361
Assets	1,571,188				1,848,711				1,571,188	1,848,711
Operating Segment | Other | Other segments
Segment Reporting Information
Net sales									434	469	1,688
Operating Segment | Other | U.S.
Segment Reporting Information
Net sales									500,496	388,546
Operating Segment | Other | U.S. | Other segments
Segment Reporting Information
Net sales											427,372
Operating Segment | Other | Non-U.S.
Segment Reporting Information
Net sales									257	11,732
Eliminations
Segment Reporting Information
Net sales									(1,664,846)	(1,627,208)	(1,994,538)
Operating income (loss)									(7,379)	7,078	(2,040)
Income (loss) before income taxes									(8,465)	6,220	(2,074)
Assets	$ (86,697)				$ (70,707)				(86,697)	(70,707)
Eliminations | Other segments
Segment Reporting Information
Net sales									$ (1,664,846)	$ (1,627,208)	$ (1,994,538)