SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Changes in Accounting Method (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Aug. 31, 2016	May 31, 2016	Feb. 29, 2016	Nov. 30, 2015	Aug. 31, 2015	May 31, 2015	Feb. 28, 2015	Nov. 30, 2014	Aug. 31, 2016	Aug. 31, 2015	Aug. 31, 2014
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of goods sold									$ 3,974,513	$ 5,311,756	$ 6,096,196
Income taxes									12,647	46,844	47,351
Earnings from continuing operations									72,543	99,131	117,606
Net earnings attributable to CMC	$ (131)	$ 19,328	$ 10,502	$ 25,063	$ 12,353	$ 28,709	$ 6,197	$ 32,184	$ 54,762	$ 79,443	$ 113,243
Basic earnings per share from continuing operations attributable to CMC (USD per share)									$ 0.63	$ 0.85	$ 1.00
Basic EPS attributable to CMC (USD per share)	$ 0	$ 0.17	$ 0.09	$ 0.22	$ 0.11	$ 0.25	$ 0.05	$ 0.27	0.48	0.68	0.96
Diluted earnings per share from continuing operations attributable to CMC (USD per share)									0.62	0.84	0.99
Diluted EPS attributable to CMC (USD per share)	$ 0.00	$ 0.17	$ 0.09	$ 0.21	$ 0.11	0.25	0.05	0.27	$ 0.47	$ 0.67	$ 0.95
Inventories, net	$ 652,754				$ 880,484				$ 652,754	$ 880,484
Current deferred tax assets	0				3,310				0	3,310
Accrued expenses and other payables	264,112				290,677				264,112	290,677
Retained earnings	$ 1,372,988				1,373,568				1,372,988	1,373,568
Net earnings									54,762	79,443	$ 113,244
Deferred income taxes									(3,889)	(13,071)	30,143
Net gain on sale of a subsidiary, cost method investment and other									(2,591)	(8,489)	(13,833)
Write-down of inventories									15,555	37,652	5,015
Inventories working capital change									209,555	127,583	(191,356)
As Originally Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of goods sold										5,213,203	6,109,338
Income taxes										83,206	42,724
Earnings from continuing operations									90,400	161,322	109,091
Net earnings attributable to CMC									$ 72,619	$ 141,634	$ 115,551
Basic earnings per share from continuing operations attributable to CMC (USD per share)									$ 0.78	$ 1.39	$ 0.93
Basic EPS attributable to CMC (USD per share)									0.63	1.22	0.98
Diluted earnings per share from continuing operations attributable to CMC (USD per share)									0.77	1.37	0.92
Diluted EPS attributable to CMC (USD per share)									$ 0.62	$ 1.20	$ 0.97
Inventories, net					781,371					$ 781,371
Current deferred tax assets					29,137					29,137
Accrued expenses and other payables					279,415					279,415
Retained earnings					$ 1,311,544					1,311,544
Net earnings										141,634	$ 115,552
Deferred income taxes										23,291	32,348
Net gain on sale of a subsidiary, cost method investment and other											(31,356)
Write-down of inventories										15,935	4,000
Inventories working capital change										50,747	(177,331)
Effect of Change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of goods sold										98,553	(13,142)
Income taxes										(36,362)	4,627
Earnings from continuing operations									$ (17,857)	(62,191)	8,515
Net earnings attributable to CMC									$ (17,857)	$ (62,191)	$ (2,308)
Basic earnings per share from continuing operations attributable to CMC (USD per share)									$ (0.15)	$ (0.54)	$ 0.07
Basic EPS attributable to CMC (USD per share)					$ 0.16	(0.24)	(0.41)	(0.04)	(0.15)	(0.54)	(0.02)
Diluted earnings per share from continuing operations attributable to CMC (USD per share)									(0.15)	(0.53)	0.07
Diluted EPS attributable to CMC (USD per share)					$ 0.16	$ (0.24)	$ (0.41)	$ (0.03)	$ (0.15)	$ (0.53)	$ (0.02)
Inventories, net					$ 99,113					$ 99,113
Current deferred tax assets					(25,827)					(25,827)
Accrued expenses and other payables					11,262					11,262
Retained earnings					$ 62,024					62,024
Net earnings										(62,191)	$ (2,308)
Deferred income taxes										(36,362)	(2,205)
Net gain on sale of a subsidiary, cost method investment and other											17,523
Write-down of inventories										21,717	1,015
Inventories working capital change										$ 76,836	$ (14,025)