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Long-Term Debt (Components of Long-Term Debt) (Details) - USD ($)
$ in Thousands
Oct. 31, 2018
Apr. 30, 2018
Oct. 31, 2017
Debt Instrument [Line Items]      
Capital lease obligation   $ 5,628 $ 6,125
Debt issuance costs and discounts $ (8,672) (9,993) (11,293)
Long-term debt including current portion 1,491,328 1,495,635 1,494,832
Less: Current portion   (1,026) (1,004)
Long-term debt excluding current portion $ 1,491,328 1,494,609 1,493,828
Senior Notes | Senior Notes, 4.125%, due October 2020      
Debt Instrument [Line Items]      
Interest rate 4.125%    
Senior notes $ 650,000 650,000 650,000
Senior Notes | Senior Notes, 5.500%, due November 2022      
Debt Instrument [Line Items]      
Interest rate 5.50%    
Senior notes $ 500,000 500,000 500,000
Senior Notes | Senior Notes, 5.250%, due October 2025      
Debt Instrument [Line Items]      
Interest rate 5.25%    
Senior notes $ 350,000 $ 350,000 $ 350,000