Segment Information - Results (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information
Net sales	$ 2,350,196	$ 2,526,845	$ 2,770,515	$ 2,817,435	$ 2,903,892	$ 3,223,547	$ 3,090,525	$ 2,603,875	$ 10,464,991	$ 11,821,839	$ 9,538,797
Operating income (loss)	181,961	$ 228,045	$ 285,032	$ 291,842	365,553	$ 531,572	$ 501,887	$ 323,397	986,880	1,722,409	1,066,881
Income (loss) before income taxes									875,337	1,619,774	935,235
Depreciation and amortization									321,082	317,198	298,999
Capital expenditures									451,945	239,390	164,935
Assets	8,275,765				7,703,563				8,275,765	7,703,563
U.S.
Segment Reporting Information
Net sales									9,927,874	11,120,013	9,014,974
Non-U.S.
Segment Reporting Information
Net sales									537,117	701,826	523,823
Operating Segment | Steel Operations
Segment Reporting Information
Net sales									8,234,179	9,263,152	7,175,409
Operating income (loss)									1,030,554	1,839,852	1,098,630
Income (loss) before income taxes									963,514	1,770,888	1,014,863
Depreciation and amortization									251,568	248,765	227,752
Capital expenditures									385,058	189,208	128,749
Assets	5,171,026				5,150,298				5,171,026	5,150,298
Operating Segment | Steel Operations | Other segments
Segment Reporting Information
Net sales									329,627	342,433	243,646
Operating Segment | Steel Operations | U.S.
Segment Reporting Information
Net sales									7,606,471	8,476,104	6,613,944
Operating Segment | Steel Operations | Non-U.S.
Segment Reporting Information
Net sales									298,081	444,615	317,819
Operating Segment | Metals Recycling Operations
Segment Reporting Information
Net sales									2,494,014	3,202,214	2,813,004
Operating income (loss)									16,308	75,891	71,052
Income (loss) before income taxes									11,432	69,861	64,638
Depreciation and amortization									46,538	46,015	48,830
Capital expenditures									46,959	35,518	25,998
Assets	940,250				946,689				940,250	946,689
Operating Segment | Metals Recycling Operations | Other segments
Segment Reporting Information
Net sales									1,296,007	1,649,614	1,402,963
Operating Segment | Metals Recycling Operations | U.S.
Segment Reporting Information
Net sales									972,200	1,295,514	1,204,188
Operating Segment | Metals Recycling Operations | Non-U.S.
Segment Reporting Information
Net sales									225,807	257,086	205,853
Operating Segment | Steel Fabrication Operations
Segment Reporting Information
Net sales									963,259	921,951	824,425
Operating income (loss)									119,099	61,901	87,295
Income (loss) before income taxes									114,359	56,236	81,046
Depreciation and amortization									11,768	11,553	11,507
Capital expenditures									12,446	8,303	7,506
Assets	386,485				415,708				386,485	415,708
Operating Segment | Steel Fabrication Operations | Other segments
Segment Reporting Information
Net sales									1,105	803	644
Operating Segment | Steel Fabrication Operations | U.S.
Segment Reporting Information
Net sales									960,657	921,023	823,630
Operating Segment | Steel Fabrication Operations | Non-U.S.
Segment Reporting Information
Net sales									1,497	125	151
Operating Segment | Other
Segment Reporting Information
Net sales									400,747	429,060	375,511
Operating income (loss)									(186,159)	(253,195)	(190,785)
Income (loss) before income taxes									(220,188)	(275,137)	(226,001)
Depreciation and amortization									11,208	10,865	10,910
Capital expenditures									7,482	6,361	2,682
Assets	1,848,711				1,283,539				1,848,711	1,283,539
Operating Segment | Other | Other segments
Segment Reporting Information
Net sales									469	1,688	2,299
Operating Segment | Other | U.S.
Segment Reporting Information
Net sales									388,546	427,372	373,212
Operating Segment | Other | Non-U.S.
Segment Reporting Information
Net sales									11,732
Eliminations
Segment Reporting Information
Net sales									(1,627,208)	(1,994,538)	(1,649,552)
Operating income (loss)									7,078	(2,040)	689
Income (loss) before income taxes									6,220	(2,074)	689
Assets	$ (70,707)				$ (92,671)				(70,707)	(92,671)
Eliminations | Other segments
Segment Reporting Information
Net sales									$ (1,627,208)	$ (1,994,538)	$ (1,649,552)