Note 21 - Unaudited Quarterly Financial Data - Unaudited Quarterly Results (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
Revenues	$ 345,861		$ 339,911		$ 335,231		$ 329,141		$ 326,642		$ 312,120		$ 294,354		$ 288,063		$ 1,350,145	[1]	$ 1,221,179	[1]	$ 1,127,092	[1]
Cost of services	169,467		165,073		163,232		161,524		161,269		146,426		131,635		127,250		659,296		566,580		487,409
Selling, general and administrative	88,038		81,573		76,514		78,519		82,945		78,052		70,973		70,494		324,645		302,464		290,007
Operating income	88,356		93,265		95,485		89,098		82,428		87,642		91,746		90,319		366,204		352,135		349,676
Net income	$ 68,823		$ 74,746		$ 53,137		$ 70,379		$ 59,552		$ 65,414		$ 66,710		$ 66,583		$ 267,085		$ 258,259		$ 338,815
Diluted EPS(1) (in dollars per share)	$ 1.77	[2]	$ 1.91	[2]	$ 1.33	[2]	$ 1.77	[2]	$ 1.52	[2]	$ 1.66	[2]	$ 1.68	[2]	$ 1.66	[2]	$ 6.78		$ 6.51		$ 8.19
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

[1] Revenues are attributed to countries based on the location of the client.

[2] 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.