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Convertible Senior Notes, Net - Schedule of Interest Expense Recognized Related to Convertible Senior Notes (Detail) - USD ($)
3 Months Ended 6 Months Ended
Jul. 31, 2019
Jul. 31, 2018
Jul. 31, 2019
Jul. 31, 2018
2018 Notes        
Debt Instrument [Line Items]        
Contractual interest expense   $ 540,000   $ 1,196,000
Interest cost related to amortization of debt issuance costs $ 0 289,000 $ 0 641,000
Interest cost related to amortization of the debt discount   3,567,000   7,850,000
2020 Notes        
Debt Instrument [Line Items]        
Contractual interest expense 938,000 938,000 1,875,000 1,875,000
Interest cost related to amortization of debt issuance costs 168,000 169,000 337,000 337,000
Interest cost related to amortization of the debt discount 2,768,000 2,601,000 5,493,000 5,162,000
2022 Notes        
Debt Instrument [Line Items]        
Contractual interest expense 719,000 719,000 1,438,000 1,438,000
Interest cost related to amortization of debt issuance costs 715,000 715,000 1,429,000 1,429,000
Interest cost related to amortization of the debt discount $ 10,650,000 $ 10,175,000 $ 21,179,000 $ 20,236,000