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		6 Months Ended
	Feb. 28, 2018	Feb. 28, 2017	Feb. 28, 2018	Feb. 28, 2017
Income available to common stockholders, net income (numerator)	$ 53,137	$ 66,710	$ 123,516	$ 133,293
Basic weighted average common shares (in shares)	38,991	39,489	39,038	39,659
Basic earnings per common share (in dollars per share)	$ 1.36	$ 1.69	$ 3.16	$ 3.36
Dilutive effect of stock options and restricted stock, weighted average common share (denominator) (in shares)	855	211	725	241
Income available to common stockholders plus assumed conversions, net income (numerator)	$ 53,137	$ 66,710	$ 123,516	$ 133,293
Diluted weighted average common shares (in shares)	39,846	39,700	39,763	39,900
Diluted earnings per common share (in dollars per share)	$ 1.33	$ 1.68	$ 3.11	$ 3.34