Supplemental information - Summary of Dividends per Share (Details) - $ / shares	3 Months Ended						9 Months Ended
	May 31, 2020	Feb. 29, 2020	Nov. 30, 2019	May 31, 2019	Feb. 28, 2019	Nov. 30, 2018	May 31, 2020	May 31, 2019
Cash and Cash Equivalents [Abstract]
Cash dividends declared (in dollars per share)	$ 0.4575	$ 0.4575	$ 0.4575	$ 0.4400	$ 0.4400	$ 0.4400	$ 1.37	$ 1.32