Segment Information - Results (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Segment Reporting Information
Net sales	$ 2,368,216	$ 1,741,301
Operating income (loss)	334,566	131,965
Income (loss) before income taxes	304,252	96,714
Depreciation and amortization	75,057	73,985
Capital expenditures	41,677	27,708
Assets	6,704,035		$ 6,423,732
U.S.
Segment Reporting Information
Net sales	2,227,567	1,648,393
Non-U.S.
Segment Reporting Information
Net sales	140,649	92,908
Steel Operations
Segment Reporting Information
Net sales	1,775,676	1,258,388
Operating income (loss)	348,532	132,275
Income (loss) before income taxes	326,764	109,375
Depreciation and amortization	56,331	52,483
Capital expenditures	33,578	23,904
Assets	4,274,952
Steel Operations | Other segments
Segment Reporting Information
Net sales	54,343	41,212
Steel Operations | U.S.
Segment Reporting Information
Net sales	1,633,630	1,156,969
Steel Operations | Non-U.S.
Segment Reporting Information
Net sales	87,703	60,207
Metals Recycling Operations
Segment Reporting Information
Net sales	720,137	487,185
Operating income (loss)	17,849	2,767
Income (loss) before income taxes	16,072	(223)
Depreciation and amortization	13,035	14,580
Capital expenditures	6,776	3,080
Assets	1,010,993
Metals Recycling Operations | Other segments
Segment Reporting Information
Net sales	356,301	217,778
Metals Recycling Operations | U.S.
Segment Reporting Information
Net sales	310,951	236,757
Metals Recycling Operations | Non-U.S.
Segment Reporting Information
Net sales	52,885	32,650
Steel Fabrication Operations
Segment Reporting Information
Net sales	194,108	180,081
Operating income (loss)	23,726	32,016
Income (loss) before income taxes	22,339	30,016
Depreciation and amortization	2,971	2,821
Capital expenditures	1,151	604
Assets	354,256
Steel Fabrication Operations | Other segments
Segment Reporting Information
Net sales	12	26
Steel Fabrication Operations | U.S.
Segment Reporting Information
Net sales	194,035	180,041
Steel Fabrication Operations | Non-U.S.
Segment Reporting Information
Net sales	61	14
Other
Segment Reporting Information
Net sales	89,285	75,889
Operating income (loss)	(53,970)	(31,930)
Income (loss) before income taxes	(59,352)	(39,291)
Depreciation and amortization	2,720	4,152
Capital expenditures	172	120
Assets	1,202,678
Other | Other segments
Segment Reporting Information
Net sales	334	1,226
Other | U.S.
Segment Reporting Information
Net sales	88,951	74,626
Other | Non-U.S.
Segment Reporting Information
Net sales		37
Eliminations
Segment Reporting Information
Net sales	(410,990)	(260,242)
Operating income (loss)	(1,571)	(3,163)
Income (loss) before income taxes	(1,571)	(3,163)
Depreciation and amortization		(51)
Assets	(138,844)
Eliminations | Other segments
Segment Reporting Information
Net sales	$ (410,990)	$ (260,242)