Segment Information - Results (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information
Net sales	$ 2,526,845	$ 3,223,547	$ 8,114,795	$ 8,917,947
Operating income (loss)	228,045	531,572	804,919	1,356,856
Income (loss) before income taxes	201,251	507,115	725,274	1,278,489
Depreciation and amortization	79,470	81,383	240,555	236,638
Capital expenditures	154,131	70,668	293,687	176,477
Assets	7,943,247		7,943,247		$ 7,703,563
U.S.
Segment Reporting Information
Net sales		3,085,224	7,701,827	8,357,030
Non-U.S.
Segment Reporting Information
Net sales		138,323	412,968	560,917
Operating Segment | Other segments
Segment Reporting Information
Net sales	(380,414)
Operating Segment | U.S.
Segment Reporting Information
Net sales	2,392,012
Operating Segment | Non-U.S.
Segment Reporting Information
Net sales	134,833
Operating Segment | Steel Operations
Segment Reporting Information
Net sales	2,008,205	2,586,626	6,396,555	7,004,088
Operating income (loss)	234,683	573,848	835,172	1,441,904
Income (loss) before income taxes	217,699	557,870	784,873	1,390,074
Depreciation and amortization	63,170	64,088	188,832	184,998
Capital expenditures	40,818	57,074	128,069	137,484
Assets	5,119,419		5,119,419
Operating Segment | Steel Operations | Other segments
Segment Reporting Information
Net sales	85,678	111,582	243,107	281,828
Operating Segment | Steel Operations | U.S.
Segment Reporting Information
Net sales	1,838,682	2,400,329	5,918,189	6,360,177
Operating Segment | Steel Operations | Non-U.S.
Segment Reporting Information
Net sales	83,845	74,715	235,259	362,083
Operating Segment | Metals Recycling Operations
Segment Reporting Information
Net sales	574,908	800,014	1,968,719	2,436,875
Operating income (loss)	(101)	14,674	24,480	62,027
Income (loss) before income taxes	(1,159)	13,488	20,846	57,458
Depreciation and amortization	11,769	11,491	34,733	34,602
Capital expenditures	11,498	11,603	30,313	27,496
Assets	972,493		972,493
Operating Segment | Metals Recycling Operations | Other segments
Segment Reporting Information
Net sales	293,999	412,795	1,013,574	1,236,830
Operating Segment | Metals Recycling Operations | U.S.
Segment Reporting Information
Net sales	230,086	323,611	778,637	1,001,267
Operating Segment | Metals Recycling Operations | Non-U.S.
Segment Reporting Information
Net sales	50,823	63,608	176,508	198,778
Operating Segment | Steel Fabrication Operations
Segment Reporting Information
Net sales	246,716	250,745	716,809	670,344
Operating income (loss)	35,280	13,104	86,567	47,039
Income (loss) before income taxes	34,080	11,801	82,897	42,898
Depreciation and amortization	2,941	2,941	8,882	8,785
Capital expenditures	3,759	1,395	8,771	5,526
Assets	414,835		414,835
Operating Segment | Steel Fabrication Operations | Other segments
Segment Reporting Information
Net sales	637	120	826	788
Operating Segment | Steel Fabrication Operations | U.S.
Segment Reporting Information
Net sales	245,914	250,625	714,782	669,500
Operating Segment | Steel Fabrication Operations | Non-U.S.
Segment Reporting Information
Net sales	165		1,201	56
Operating Segment | Other
Segment Reporting Information
Net sales	77,430	111,953	290,688	327,651
Operating income (loss)	(42,441)	(67,113)	(148,514)	(186,137)
Income (loss) before income taxes	(49,780)	(73,109)	(169,914)	(203,970)
Depreciation and amortization	1,590	2,863	8,108	8,253
Capital expenditures	98,056	596	126,534	5,971
Assets	1,513,798		1,513,798
Operating Segment | Other | Other segments
Segment Reporting Information
Net sales	100	1,294	469	1,565
Operating Segment | Other | U.S.
Segment Reporting Information
Net sales	77,330	110,659	290,219	326,086
Eliminations
Segment Reporting Information
Net sales	(380,414)	(525,791)	(1,257,976)	(1,521,011)
Operating income (loss)	624	(2,941)	7,214	(7,977)
Income (loss) before income taxes	411	(2,935)	6,572	(7,971)
Assets	$ (77,298)		(77,298)
Eliminations | Other segments
Segment Reporting Information
Net sales		$ (525,791)	$ (1,257,976)	$ (1,521,011)