Segment Information - Results (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Segment Reporting Information
Net sales	$ 2,094,305	$ 2,770,515	$ 4,669,405	$ 5,587,950
Operating income (loss)	158,850	285,032	432,536	576,874
Income (loss) before income taxes	103,045	256,960	351,301	524,023
Depreciation and amortization	78,721	80,911	158,980	161,085
Capital expenditures	309,716	85,120	527,251	139,556
Assets	8,468,341		8,468,341		$ 8,275,765
U.S.
Segment Reporting Information
Net sales	2,014,295	2,643,662	4,450,990	5,315,555
Non-U.S.
Segment Reporting Information
Net sales	80,010	126,853	218,415	272,395
Operating Segment | Steel Operations
Segment Reporting Information
Net sales	1,699,398	2,188,185	3,715,550	4,388,350
Operating income (loss)	168,043	291,411	456,437	600,489
Income (loss) before income taxes	151,012	274,155	424,567	567,174
Depreciation and amortization	60,686	63,150	123,113	125,662
Capital expenditures	301,179	43,575	494,452	87,251
Assets	5,533,365		5,533,365
Operating Segment | Steel Operations | Other segments
Segment Reporting Information
Net sales	71,371	81,834	145,817	157,429
Operating Segment | Steel Operations | U.S.
Segment Reporting Information
Net sales	1,578,252	2,040,756	3,438,837	4,085,247
Operating Segment | Steel Operations | Non-U.S.
Segment Reporting Information
Net sales	49,775	65,595	130,896	145,674
Operating Segment | Metals Recycling Operations
Segment Reporting Information
Net sales	416,688	656,766	1,045,427	1,393,811
Operating income (loss)	(8,715)	7,619	(3,187)	24,581
Income (loss) before income taxes	(12,654)	6,500	(8,349)	22,005
Depreciation and amortization	12,170	11,525	24,142	22,964
Capital expenditures	3,821	12,173	22,044	18,815
Assets	867,866		867,866
Operating Segment | Metals Recycling Operations | Other segments
Segment Reporting Information
Net sales	260,106	333,667	596,988	719,575
Operating Segment | Metals Recycling Operations | U.S.
Segment Reporting Information
Net sales	126,818	262,826	361,651	548,551
Operating Segment | Metals Recycling Operations | Non-U.S.
Segment Reporting Information
Net sales	29,764	60,273	86,788	125,685
Operating Segment | Steel Fabrication Operations
Segment Reporting Information
Net sales	218,428	241,424	439,869	470,093
Operating income (loss)	27,155	30,664	56,318	51,287
Income (loss) before income taxes	25,959	29,466	53,877	48,817
Depreciation and amortization	2,737	2,974	5,485	5,941
Capital expenditures	2,364	3,019	6,453	5,012
Assets	397,070		397,070
Operating Segment | Steel Fabrication Operations | Other segments
Segment Reporting Information
Net sales	3,177		3,683	189
Operating Segment | Steel Fabrication Operations | U.S.
Segment Reporting Information
Net sales	214,951	240,439	435,886	468,868
Operating Segment | Steel Fabrication Operations | Non-U.S.
Segment Reporting Information
Net sales	300	985	300	1,036
Operating Segment | Other
Segment Reporting Information
Net sales	94,445	99,762	215,135	213,258
Operating income (loss)	(32,089)	(49,153)	(78,444)	(106,073)
Income (loss) before income taxes	(65,445)	(57,438)	(119,685)	(120,134)
Depreciation and amortization	3,128	3,262	6,240	6,518
Capital expenditures	2,352	26,353	4,302	28,478
Assets	1,718,951		1,718,951
Operating Segment | Other | Other segments
Segment Reporting Information
Net sales		121	88	369
Operating Segment | Other | U.S.
Segment Reporting Information
Net sales	94,274	99,641	214,616	212,889
Operating Segment | Other | Non-U.S.
Segment Reporting Information
Net sales	171		431
Eliminations
Segment Reporting Information
Net sales	(334,654)	(415,622)	(746,576)	(877,562)
Operating income (loss)	4,456	4,491	1,412	6,590
Income (loss) before income taxes	4,173	4,277	891	6,161
Assets	(48,911)		(48,911)
Eliminations | Other segments
Segment Reporting Information
Net sales	$ (334,654)	$ (415,622)	$ (746,576)	$ (877,562)