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Fair Value Measurements - Additional Information (Details) - USD ($)
$ in Thousands
Oct. 31, 2019
Jan. 31, 2019
Jan. 31, 2018
0.125% Convertible Senior Notes Due 2025      
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]      
Debt instrument, Principal amount [1] $ 805,000    
Debt instrument, Interest rate 0.125%    
0.125% Convertible Senior Notes Due 2025 | Level 2      
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]      
Estimated fair value of convertible notes $ 922,500    
0.375% Convertible Senior Notes Due 2023      
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]      
Debt instrument, Principal amount $ 230,000 $ 230,000 $ 230,000
Debt instrument, Interest rate 0.375%   0.375%
0.375% Convertible Senior Notes Due 2023 | Level 2      
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]      
Estimated fair value of convertible notes $ 714,600 $ 428,400  
[1] The 2025 Notes were issued on June 11, 2019.