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Share Repurchase Plan (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended 24 Months Ended
Aug. 31, 2018
Aug. 31, 2017
Aug. 31, 2016
Aug. 31, 2018
Sep. 01, 2018
Sep. 01, 2016
Equity, Class of Treasury Stock [Line Items]            
Total cost of repurchased shares $ 22,616 $ 31,109 $ 32,131      
2016 To 2018 Share Repurchase Program [Member]            
Equity, Class of Treasury Stock [Line Items]            
Share buy-back plan, amount authorized $ 75,000     $ 75,000    
Share buy-back plan, number of shares repurchased 175,306     465,879    
Average price of shares repurchased $ 128.99          
Total cost of repurchased shares $ 22,600     $ 53,700    
Maximum [Member] | 2016 To 2018 Share Repurchase Program [Member]            
Equity, Class of Treasury Stock [Line Items]            
Share buy-back plan, amount authorized           $ 75,000
Subsequent Events [Member] | Maximum [Member] | 2018 To 2020 Share Repurchase Program [Member]            
Equity, Class of Treasury Stock [Line Items]            
Share buy-back plan, amount authorized         $ 75,000