Segment Information - Results (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information
Net sales	$ 2,336,479	$ 2,443,382	$ 2,390,720	$ 2,368,216	$ 1,910,596	$ 2,101,310	$ 2,023,902	$ 1,741,301	$ 9,538,797	$ 7,777,109	$ 7,594,411
Operating income (loss)	195,947	$ 271,015	$ 265,353	$ 334,566	56,024	$ 283,855	$ 256,122	$ 131,965	1,066,881	727,966	(72,784)
Income (loss) before income taxes									935,235	564,133	(242,117)
Depreciation and amortization									298,999	296,109	294,595
Capital expenditures									164,935	198,160	114,501
Assets	6,855,732				6,423,732				6,855,732	6,423,732
U.S.
Segment Reporting Information
Net sales									9,014,974	7,379,736	7,139,898
Non-U.S.
Segment Reporting Information
Net sales									523,823	397,373	454,513
Steel Operations
Segment Reporting Information
Net sales									7,175,409	5,870,924	5,422,475
Operating income (loss)									1,098,630	926,954	403,216
Income (loss) before income taxes									1,014,863	836,818	312,402
Depreciation and amortization									227,752	213,227	205,827
Capital expenditures									128,749	160,767	71,752
Assets	4,257,167				4,126,448				4,257,167	4,126,448
Steel Operations | Other segments
Segment Reporting Information
Net sales									243,646	236,213	218,963
Steel Operations | U.S.
Segment Reporting Information
Net sales									6,613,944	5,399,275	4,935,549
Steel Operations | Non-U.S.
Segment Reporting Information
Net sales									317,819	235,436	267,963
Metals Recycling Operations
Segment Reporting Information
Net sales									2,813,004	2,171,877	2,337,716
Operating income (loss)									71,052	20,423	(448,137)
Income (loss) before income taxes									64,638	8,378	(465,426)
Depreciation and amortization									48,830	56,483	65,640
Capital expenditures									25,998	29,443	21,874
Assets	1,011,298				1,038,946				1,011,298	1,038,946
Metals Recycling Operations | Other segments
Segment Reporting Information
Net sales									1,402,963	1,002,536	904,713
Metals Recycling Operations | U.S.
Segment Reporting Information
Net sales									1,204,188	1,007,781	1,249,061
Metals Recycling Operations | Non-U.S.
Segment Reporting Information
Net sales									205,853	161,560	183,942
Steel Fabrication Operations
Segment Reporting Information
Net sales									824,425	703,522	673,399
Operating income (loss)									87,295	90,955	115,947
Income (loss) before income taxes									81,046	82,776	108,795
Depreciation and amortization									11,507	10,952	9,587
Capital expenditures									7,506	3,123	3,395
Assets	384,402				340,902				384,402	340,902
Steel Fabrication Operations | Other segments
Segment Reporting Information
Net sales									644	2,481	25
Steel Fabrication Operations | U.S.
Segment Reporting Information
Net sales									823,630	700,852	671,323
Steel Fabrication Operations | Non-U.S.
Segment Reporting Information
Net sales									151	189	2,051
Other
Segment Reporting Information
Net sales									375,511	276,912	314,847
Operating income (loss)									(190,785)	(301,116)	(148,784)
Income (loss) before income taxes									(226,001)	(354,589)	(202,862)
Depreciation and amortization									10,910	15,652	13,746
Capital expenditures									2,682	4,827	17,480
Assets	1,334,361				1,054,448				1,334,361	1,054,448
Other | Other segments
Segment Reporting Information
Net sales									2,299	4,896	30,325
Other | U.S.
Segment Reporting Information
Net sales									373,212	271,828	283,965
Other | Non-U.S.
Segment Reporting Information
Net sales										188	557
Eliminations
Segment Reporting Information
Net sales									(1,649,552)	(1,246,126)	(1,154,026)
Operating income (loss)									689	(9,250)	4,974
Income (loss) before income taxes									689	(9,250)	4,974
Depreciation and amortization										(205)	(205)
Assets	$ (131,496)				$ (137,012)				(131,496)	(137,012)
Eliminations | Other segments
Segment Reporting Information
Net sales									$ (1,649,552)	$ (1,246,126)	$ (1,154,026)