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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 9 Months Ended
Oct. 31, 2018
Oct. 31, 2017
Oct. 31, 2018
Oct. 31, 2017
2018 Notes        
Debt Instrument [Line Items]        
Contractual interest expense $ 0 $ 656 $ 1,196 $ 1,969
Interest cost related to amortization of debt issuance costs 0 352 641 1,056
Interest cost related to amortization of the debt discount 0 4,162 7,850 12,310
2020 Notes        
Debt Instrument [Line Items]        
Contractual interest expense 937 938 2,812 2,813
Interest cost related to amortization of debt issuance costs 167 168 504 503
Interest cost related to amortization of the debt discount 2,643 2,482 7,805 7,332
2022 Notes        
Debt Instrument [Line Items]        
Contractual interest expense 718 367 2,156 367
Interest cost related to amortization of debt issuance costs 714 365 2,143 365
Interest cost related to amortization of the debt discount $ 10,293 $ 5,042 $ 30,529 $ 5,042