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Convertible Senior Notes, Net - Schedule of Interest Expense Recognized Related to Convertible Senior Notes (Detail) - USD ($)
$ in Thousands
3 Months Ended
Apr. 30, 2019
Apr. 30, 2018
2018 Notes    
Debt Instrument [Line Items]    
Contractual interest expense $ 0 $ 656
Interest cost related to amortization of debt issuance costs 0 352
Interest cost related to amortization of the debt discount 0 4,283
2020 Notes    
Debt Instrument [Line Items]    
Contractual interest expense 937 937
Interest cost related to amortization of debt issuance costs 169 168
Interest cost related to amortization of the debt discount 2,725 2,561
2022 Notes    
Debt Instrument [Line Items]    
Contractual interest expense 719 719
Interest cost related to amortization of debt issuance costs 714 700
Interest cost related to amortization of the debt discount $ 10,529 $ 10,061