Note 11 - Common Stock and Earnings Per Share - Weighted Average Shares Outstanding (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	May 31, 2019	May 31, 2018	May 31, 2019	May 31, 2018
Income available to common stockholders, net income (numerator)	$ 92,265	$ 74,746	$ 261,263	$ 198,262
Basic weighted average common shares (in shares)	38,223	38,594	38,128	38,890
Income available to common stockholders, per share amount (in dollars per share)	$ 2.41	$ 1.94	$ 6.85	$ 5.10
Dilutive effect of stock options and restricted stock, weighted average common share (denominator) (in shares)	770	510	679	653
Income available to common stockholders plus assumed conversions, net income (numerator)	$ 92,265	$ 74,746	$ 261,263	$ 198,262
Diluted weighted average common shares (in shares)	38,993	39,104	38,807	39,543
Income available to common stockholders plus assumed conversions, per share amount (in dollars per share)	$ 2.37	$ 1.91	$ 6.73	$ 5.01
Basic earnings per common share (in dollars per share)	2.41	1.94	6.85	5.10
Diluted earnings per common share (in dollars per share)	$ 2.37	$ 1.91	$ 6.73	$ 5.01