Note 13 - Common Stock and 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, 2018		May 31, 2018		Feb. 28, 2018		Nov. 30, 2017		Aug. 31, 2017		May 31, 2017		Feb. 28, 2017		Nov. 30, 2016		Aug. 31, 2018	Aug. 31, 2017	Aug. 31, 2016
Income available to common stockholders, net income (numerator)																	$ 267,085	$ 258,259	$ 338,815
Basic weighted average common shares (in shares)																	38,733	39,444	40,880
Income available to common stockholders, per share amount (in dollars per share)																	$ 6.90	$ 6.55	$ 8.29
Dilutive effect of stock options and restricted stock, weighted average common share (denominator) (in shares)																	644	198	485
Income available to common stockholders plus assumed conversions, net income (numerator)																	$ 267,085	$ 258,259	$ 338,815
Diluted weighted average common shares (in shares)	38,879		39,104		39,846		39,680		39,281		39,457		39,700		40,100		39,377	39,642	41,365
Diluted EPS(1) (in dollars per share)	$ 1.77	[1]	$ 1.91	[1]	$ 1.33	[1]	$ 1.77	[1]	$ 1.52	[1]	$ 1.66	[1]	$ 1.68	[1]	$ 1.66	[1]	$ 6.78	$ 6.51	$ 8.19

[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.