REVENUE RECOGNITION - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	May 31, 2019	May 31, 2018	May 31, 2019	May 31, 2018
Disaggregation of Revenue [Line Items]
Net sales	$ 1,605,872	$ 1,204,484	$ 4,285,997	$ 3,335,285
Steel Products
Disaggregation of Revenue [Line Items]
Net sales	1,256,266	802,186	3,272,073	2,203,194
Ferrous Scrap
Disaggregation of Revenue [Line Items]
Net sales	115,847	153,213	351,170	408,935
Nonferrous Scrap
Disaggregation of Revenue [Line Items]
Net sales	128,873	155,820	388,905	470,221
Construction Materials
Disaggregation of Revenue [Line Items]
Net sales	71,228	70,436	188,589	180,641
Other
Disaggregation of Revenue [Line Items]
Net sales	33,658	22,829	85,260	72,294
Americas Recycling
Disaggregation of Revenue [Line Items]
Net sales	228,786	292,679	694,855	832,448
Americas Recycling | Steel Products
Disaggregation of Revenue [Line Items]
Net sales	238	319	679	894
Americas Recycling | Ferrous Scrap
Disaggregation of Revenue [Line Items]
Net sales	106,404	144,398	323,311	383,358
Americas Recycling | Nonferrous Scrap
Disaggregation of Revenue [Line Items]
Net sales	121,581	147,683	369,660	447,060
Americas Recycling | Construction Materials
Disaggregation of Revenue [Line Items]
Net sales	0	0	0	0
Americas Recycling | Other
Disaggregation of Revenue [Line Items]
Net sales	563	279	1,205	1,136
Americas Mills
Disaggregation of Revenue [Line Items]
Net sales	536,035	332,459	1,382,501	841,895
Americas Mills | Steel Products
Disaggregation of Revenue [Line Items]
Net sales	501,925	305,390	1,294,217	766,976
Americas Mills | Ferrous Scrap
Disaggregation of Revenue [Line Items]
Net sales	8,916	8,462	26,802	24,605
Americas Mills | Nonferrous Scrap
Disaggregation of Revenue [Line Items]
Net sales	4,080	4,851	10,568	12,734
Americas Mills | Construction Materials
Disaggregation of Revenue [Line Items]
Net sales	0	0	0	0
Americas Mills | Other
Disaggregation of Revenue [Line Items]
Net sales	21,114	13,756	50,914	37,580
Americas Fabrication
Disaggregation of Revenue [Line Items]
Net sales	629,510	375,183	1,590,746	1,015,934
Americas Fabrication | Steel Products
Disaggregation of Revenue [Line Items]
Net sales	554,672	303,363	1,392,442	830,172
Americas Fabrication | Ferrous Scrap
Disaggregation of Revenue [Line Items]
Net sales	2	2	2	2
Americas Fabrication | Nonferrous Scrap
Disaggregation of Revenue [Line Items]
Net sales	0	0	0	0
Americas Fabrication | Construction Materials
Disaggregation of Revenue [Line Items]
Net sales	71,228	70,436	188,589	180,641
Americas Fabrication | Other
Disaggregation of Revenue [Line Items]
Net sales	3,608	1,382	9,713	5,119
International Mill
Disaggregation of Revenue [Line Items]
Net sales	209,022	201,438	610,640	633,134
International Mill | Steel Products
Disaggregation of Revenue [Line Items]
Net sales	199,431	193,114	584,735	605,152
International Mill | Ferrous Scrap
Disaggregation of Revenue [Line Items]
Net sales	525	351	1,055	970
International Mill | Nonferrous Scrap
Disaggregation of Revenue [Line Items]
Net sales	3,212	3,286	8,677	10,427
International Mill | Construction Materials
Disaggregation of Revenue [Line Items]
Net sales	0	0	0	0
International Mill | Other
Disaggregation of Revenue [Line Items]
Net sales	5,854	4,687	16,173	16,585
Corporate
Disaggregation of Revenue [Line Items]
Net sales	2,519	2,725	7,255	11,874
Corporate | Steel Products
Disaggregation of Revenue [Line Items]
Net sales	0	0	0	0
Corporate | Ferrous Scrap
Disaggregation of Revenue [Line Items]
Net sales	0	0	0	0
Corporate | Nonferrous Scrap
Disaggregation of Revenue [Line Items]
Net sales	0	0	0	0
Corporate | Construction Materials
Disaggregation of Revenue [Line Items]
Net sales	0	0	0	0
Corporate | Other
Disaggregation of Revenue [Line Items]
Net sales	$ 2,519	$ 2,725	$ 7,255	$ 11,874