Segment Information - Results (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information
Net sales	$ 2,903,892	$ 3,223,547	$ 3,090,525	$ 2,603,875	$ 2,336,479	$ 2,443,382	$ 2,390,720	$ 2,368,216	$ 11,821,839	$ 9,538,797	$ 7,777,109
Operating income (loss)	365,553	$ 531,572	$ 501,887	$ 323,397	195,947	$ 271,015	$ 265,353	$ 334,566	1,722,409	1,066,881	727,966
Income (loss) before income taxes									1,619,774	935,235	564,133
Depreciation and amortization									317,198	298,999	296,109
Capital expenditures									239,390	164,935	198,160
Assets	7,703,563				6,855,732				7,703,563	6,855,732
U.S.
Segment Reporting Information
Net sales									11,120,013	9,014,974	7,379,736
Non-U.S.
Segment Reporting Information
Net sales									701,826	523,823	397,373
Operating Segment | Steel Operations
Segment Reporting Information
Net sales									9,263,152	7,175,409	5,870,924
Operating income (loss)									1,839,852	1,098,630	926,954
Income (loss) before income taxes									1,770,888	1,014,863	836,818
Depreciation and amortization									248,765	227,752	213,227
Capital expenditures									189,208	128,749	160,767
Assets	5,150,298				4,257,167				5,150,298	4,257,167
Operating Segment | Steel Operations | Other segments
Segment Reporting Information
Net sales									342,433	243,646	236,213
Operating Segment | Steel Operations | U.S.
Segment Reporting Information
Net sales									8,476,104	6,613,944	5,399,275
Operating Segment | Steel Operations | Non-U.S.
Segment Reporting Information
Net sales									444,615	317,819	235,436
Operating Segment | Metals Recycling Operations
Segment Reporting Information
Net sales									3,202,214	2,813,004	2,171,877
Operating income (loss)									75,891	71,052	20,423
Income (loss) before income taxes									69,861	64,638	8,378
Depreciation and amortization									46,015	48,830	56,483
Capital expenditures									35,518	25,998	29,443
Assets	946,689				1,011,298				946,689	1,011,298
Operating Segment | Metals Recycling Operations | Other segments
Segment Reporting Information
Net sales									1,649,614	1,402,963	1,002,536
Operating Segment | Metals Recycling Operations | U.S.
Segment Reporting Information
Net sales									1,295,514	1,204,188	1,007,781
Operating Segment | Metals Recycling Operations | Non-U.S.
Segment Reporting Information
Net sales									257,086	205,853	161,560
Operating Segment | Steel Fabrication Operations
Segment Reporting Information
Net sales									921,951	824,425	703,522
Operating income (loss)									61,901	87,295	90,955
Income (loss) before income taxes									56,236	81,046	82,776
Depreciation and amortization									11,553	11,507	10,952
Capital expenditures									8,303	7,506	3,123
Assets	415,708				384,402				415,708	384,402
Operating Segment | Steel Fabrication Operations | Other segments
Segment Reporting Information
Net sales									803	644	2,481
Operating Segment | Steel Fabrication Operations | U.S.
Segment Reporting Information
Net sales									921,023	823,630	700,852
Operating Segment | Steel Fabrication Operations | Non-U.S.
Segment Reporting Information
Net sales									125	151	189
Operating Segment | Other
Segment Reporting Information
Net sales									429,060	375,511	276,912
Operating income (loss)									(253,195)	(190,785)	(301,116)
Income (loss) before income taxes									(275,137)	(226,001)	(354,589)
Depreciation and amortization									10,865	10,910	15,652
Capital expenditures									6,361	2,682	4,827
Assets	1,283,539				1,334,361				1,283,539	1,334,361
Operating Segment | Other | Other segments
Segment Reporting Information
Net sales									1,688	2,299	4,896
Operating Segment | Other | U.S.
Segment Reporting Information
Net sales									427,372	373,212	271,828
Operating Segment | Other | Non-U.S.
Segment Reporting Information
Net sales											188
Eliminations
Segment Reporting Information
Net sales									(1,994,538)	(1,649,552)	(1,246,126)
Operating income (loss)									(2,040)	689	(9,250)
Income (loss) before income taxes									(2,074)	689	(9,250)
Depreciation and amortization											(205)
Assets	$ (92,671)				$ (131,496)				(92,671)	(131,496)
Eliminations | Other segments
Segment Reporting Information
Net sales									$ (1,994,538)	$ (1,649,552)	$ (1,246,126)