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Debt | Senior Notes (Details) - USD ($)
$ in Thousands
1 Months Ended 3 Months Ended
Jan. 31, 2017
Apr. 30, 2018
Apr. 30, 2017
Jan. 31, 2018
Feb. 27, 2017
Debt Instrument          
Proceeds from Issuance of Long-term Debt   $ 0 $ 1,008,148    
Senior Notes | 2017 Senior Notes          
Debt Instrument          
Proceeds from Issuance of Long-term Debt $ 989,900        
Debt Instrument, Unamortized Discount 1,600        
Debt Issuance Costs, Gross 8,500        
Senior Notes | 3.70% Senior Notes          
Debt Instrument          
Debt Instrument, Face Amount $ 500,000 500,000   $ 500,000  
Debt Instrument, Redemption Price, Discounted Scheduled Payments, Discount Rate, Basis Spread on Variable Rate         0.30%
Debt Instrument, Interest Rate, Stated Percentage 3.70%        
Senior Notes | 4.95% Senior Notes          
Debt Instrument          
Debt Instrument, Face Amount $ 500,000 $ 500,000   $ 500,000  
Debt Instrument, Redemption Price, Discounted Scheduled Payments, Discount Rate, Basis Spread on Variable Rate         0.40%
Debt Instrument, Interest Rate, Stated Percentage 4.95%