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Convertible Senior Notes, Net (Schedule of Interest Expense Recognized Related to Convertible Senior Notes) (Detail) - USD ($)
$ in Thousands
12 Months Ended
Jan. 31, 2019
Jan. 31, 2018
Jan. 31, 2017
Debt Instrument [Line Items]      
Interest cost related to amortization of the debt discount $ 59,974 $ 43,916 $ 26,947
2018 Notes      
Debt Instrument [Line Items]      
Contractual interest expense 1,196 2,625 2,625
Interest cost related to amortization of debt issuance costs 641 1,409 1,408
Interest cost related to amortization of the debt discount 7,850 16,530 15,607
2020 Notes      
Debt Instrument [Line Items]      
Contractual interest expense 3,750 3,750 3,750
Interest cost related to amortization of debt issuance costs 673 673 675
Interest cost related to amortization of the debt discount 10,488 9,852 9,257
2022 Notes      
Debt Instrument [Line Items]      
Contractual interest expense 2,875 1,086 0
Interest cost related to amortization of debt issuance costs 2,858 1,080 0
Interest cost related to amortization of the debt discount $ 40,939 $ 14,989 $ 0