Note 13 - Common Stock and Earnings Per Share - Weighted Average Shares Outstanding (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Aug. 31, 2019		May 31, 2019		Feb. 28, 2019		Nov. 30, 2018		Aug. 31, 2018		May 31, 2018		Feb. 28, 2018		Nov. 30, 2017		Aug. 31, 2019	Aug. 31, 2018	Aug. 31, 2017
Income available to common stockholders, net income (numerator)																	$ 352,790	$ 267,085	$ 258,259
Basic weighted average common shares (in shares)																	38,144,000	38,733,000	39,444,000
Income available to common stockholders, per share amount (in dollars per share)																	$ 9.25	$ 6.90	$ 6.55
Dilutive effect of stock options and restricted stock, weighted average common share (denominator) (in shares)																	729,000	644,000	198,000
Income available to common stockholders plus assumed conversions, net income (numerator)																	$ 352,790	$ 267,085	$ 258,259
Diluted weighted average common shares (in shares)	39,056		38,993		38,619		38,809		38,879,000		39,104,000		39,846,000		39,680,000		38,873,000	39,377,000	39,642,000
Diluted EPS(1) (in dollars per share)	$ 2.34	[1]	$ 2.37	[1]	$ 2.19	[1]	$ 2.17	[1]	$ 1.77	[1]	$ 1.91	[1]	$ 1.33	[1]	$ 1.77	[1]	$ 9.08	$ 6.78	$ 6.51
Basic earnings per common share (in dollars per share)																	9.25	6.90	6.55
Diluted earnings per common share (in dollars per share)	$ 2.34	[1]	$ 2.37	[1]	$ 2.19	[1]	$ 2.17	[1]	$ 1.77	[1]	$ 1.91	[1]	$ 1.33	[1]	$ 1.77	[1]	$ 9.08	$ 6.78	$ 6.51

[1] Diluted earnings per common share is calculated independently for each of the periods presented. Accordingly, the sum of the quarterly EPS amounts may not equal the total for the fiscal year.