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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, 2018
Apr. 30, 2017
2018 Notes    
Debt Instrument [Line Items]    
Contractual interest expense $ 656 $ 656
Interest cost related to amortization of debt issuance costs 352 352
Interest cost related to amortization of the debt discount 4,283 4,044
2020 Notes    
Debt Instrument [Line Items]    
Contractual interest expense 937 938
Interest cost related to amortization of debt issuance costs 168 168
Interest cost related to amortization of the debt discount 2,561 2,406
2022 Notes    
Debt Instrument [Line Items]    
Contractual interest expense 719 0
Interest cost related to amortization of debt issuance costs 714 0
Interest cost related to amortization of the debt discount $ 10,061 $ 0