Asset Retirement Obligations - Narrative (Details)	12 Months Ended
	Aug. 31, 2019
Asset Retirement Obligations [Line Items]
Estimated impact of inflation per year	3.00%
Credit adjusted risk free rates, minimum	7.00%
Credit adjusted risk free rates, maximum	7.50%
Minimum
Asset Retirement Obligations [Line Items]
Remaining lives	1 year
Maximum
Asset Retirement Obligations [Line Items]
Remaining lives	25 years