Segment Information - Results (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment Reporting Information
Net sales	$ 2,603,875	$ 2,368,216
Operating income (loss)	323,397	334,566
Income (loss) before income taxes	295,964	304,252
Depreciation and amortization	76,135	75,057
Capital expenditures	50,606	41,677
Assets	6,972,813		$ 6,855,732
U.S.
Segment Reporting Information
Net sales	2,456,083	2,227,567
Non-U.S.
Segment Reporting Information
Net sales	147,792	140,649
Steel Operations
Segment Reporting Information
Net sales	1,981,774	1,775,676
Operating income (loss)	334,562	348,532
Income (loss) before income taxes	315,805	326,764
Depreciation and amortization	59,141	56,331
Capital expenditures	38,402	33,578
Assets	4,462,706
Steel Operations | Other segments
Segment Reporting Information
Net sales	59,985	54,343
Steel Operations | U.S.
Segment Reporting Information
Net sales	1,832,303	1,633,630
Steel Operations | Non-U.S.
Segment Reporting Information
Net sales	89,486	87,703
Metals Recycling Operations
Segment Reporting Information
Net sales	752,766	720,137
Operating income (loss)	24,715	17,849
Income (loss) before income taxes	23,005	16,072
Depreciation and amortization	11,558	13,035
Capital expenditures	6,946	6,776
Assets	1,006,510
Metals Recycling Operations | Other segments
Segment Reporting Information
Net sales	364,644	356,301
Metals Recycling Operations | U.S.
Segment Reporting Information
Net sales	329,872	310,951
Metals Recycling Operations | Non-U.S.
Segment Reporting Information
Net sales	58,250	52,885
Steel Fabrication Operations
Segment Reporting Information
Net sales	201,703	194,108
Operating income (loss)	19,791	23,726
Income (loss) before income taxes	18,457	22,339
Depreciation and amortization	2,898	2,971
Capital expenditures	2,077	1,151
Assets	350,359
Steel Fabrication Operations | Other segments
Segment Reporting Information
Net sales	210	12
Steel Fabrication Operations | U.S.
Segment Reporting Information
Net sales	201,437	194,035
Steel Fabrication Operations | Non-U.S.
Segment Reporting Information
Net sales	56	61
Other
Segment Reporting Information
Net sales	92,618	89,285
Operating income (loss)	(55,406)	(53,970)
Income (loss) before income taxes	(61,033)	(59,352)
Depreciation and amortization	2,538	2,720
Capital expenditures	3,181	172
Assets	1,250,869
Other | Other segments
Segment Reporting Information
Net sales	147	334
Other | U.S.
Segment Reporting Information
Net sales	92,471	88,951
Eliminations
Segment Reporting Information
Net sales	(424,986)	(410,990)
Operating income (loss)	(265)	(1,571)
Income (loss) before income taxes	(270)	(1,571)
Assets	(97,631)
Eliminations | Other segments
Segment Reporting Information
Net sales	$ (424,986)	$ (410,990)