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Debt | Senior Notes (Details) - USD ($)
$ in Thousands
9 Months Ended
Oct. 31, 2019
Oct. 31, 2018
Jan. 31, 2019
Jan. 31, 2017
Debt Instrument        
Proceeds from Issuance of Long-term Debt $ 300,000 $ 0    
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%