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, 2019	Feb. 28, 2018	Feb. 28, 2019	Feb. 28, 2018
Income available to common stockholders, net income (numerator)	$ 84,702	$ 53,137	$ 168,998	$ 123,516
Basic weighted average common shares (in shares)	38,055	38,991	38,081	39,038
Income available to common stockholders, per share amount (in dollars per share)	$ 2.23	$ 1.36	$ 4.44	$ 3.16
Dilutive effect of stock options and restricted stock, weighted average common share (denominator) (in shares)	564	855	633	725
Income available to common stockholders plus assumed conversions, net income (numerator)	$ 84,702	$ 53,137	$ 168,998	$ 123,516
Diluted weighted average common shares (in shares)	38,619	39,846	38,714	39,763
Income available to common stockholders plus assumed conversions, per share amount (in dollars per share)	$ 2.19	$ 1.33	$ 4.37	$ 3.11
Basic earnings per common share (in dollars per share)	2.23	1.36	4.44	3.16
Diluted earnings per common share (in dollars per share)	$ 2.19	$ 1.33	$ 4.37	$ 3.11