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New Accounting Pronouncements (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended
Jul. 31, 2017
Jul. 31, 2016
Apr. 30, 2017
Accounting Changes and Error Corrections [Abstract]      
Excess tax benefit related to stock-based compensation $ 2,200 $ 900  
New Accounting Pronouncement, Early Adoption [Line Items]      
Reclassification from operating activities (26,570) (32,947)  
Reclassification to financing activities $ (6,773) $ (4,276)  
Basic (in usd per share) $ 1.37 $ 1.33  
Diluted (in usd per share) $ 1.36 $ 1.32  
Accounting Standards Update 2016-09 [Member] | New Accounting Pronouncement, Early Adoption, Effect [Member]      
New Accounting Pronouncement, Early Adoption [Line Items]      
Reclassification to financing activities   $ 2,600  
Accounting Standards Update 2015-03 [Member] | Long-term Debt [Member]      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Reclassification of debt issuance costs     $ 300