Earnings Per Share - Weighted Average Shares Outstanding (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Aug. 31, 2020	May 31, 2020	Feb. 28, 2020	Nov. 30, 2019	Aug. 31, 2019	May 31, 2019	Feb. 28, 2019	Nov. 30, 2018	Aug. 31, 2020	Aug. 31, 2019	Aug. 31, 2018
Basic EPS
Income available to common stockholders, Net Income (Numerator)									$ 372,938	$ 352,790	$ 267,085
Income available to common stockholders, Weighted Average Common Shares (Denominator) (in shares)									37,936	38,144	38,733
Income available to common stockholders, Per Share Amount (in dollars per share)									$ 9.83	$ 9.25	$ 6.90
Diluted EPS
Income available to common stockholders plus assumed conversions, Net Income (Numerator)									$ 372,938	$ 352,790	$ 267,085
Dilutive effect of stock options and restricted stock (in shares)									710	729	644
Income available to common stockholders plus assumed conversions, Weighted Average Common Shares (Denominator) (in shares)	38,940	38,481	38,576	38,587	39,056	38,993	38,619	38,809	38,646	38,873	39,377
Income available to common stockholders plus assumed conversions, Per Share Amount (in dollars per share)	$ 2.29	$ 2.63	$ 2.30	$ 2.43	$ 2.34	$ 2.37	$ 2.19	$ 2.17	$ 9.65	$ 9.08	$ 6.78