FAIR VALUE (Narrative) (Details) $ in Millions	3 Months Ended	6 Months Ended
Feb. 28, 2018 USD ($)	Feb. 28, 2018 levels
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of fair value hierarchy \| levels		3
Structural Steel Fabrication Operations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Non-recurring impairment charge \| $	$ 12.1

This represents number of levels used to prioritize the inputs to valuation techniques used to measure fair value. The fair value hierarchy gives the highest priority to quoted prices (unadjusted) in active markets for identical assets or liabilities (Level 1), inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly (Level 2), and the lowest priority to unobservable inputs (Level 3). Where the quoted price in an active market for the identical liability is not available, the Level 1 input is the quoted price of an identical liability when traded as an asset.

No definition available.

Name:	cmc_NumberofFairValueHierarchy
Namespace Prefix:	cmc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration