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Convertible Senior Notes, Net - Schedule of Senior Notes (Detail) - USD ($)
3 Months Ended 6 Months Ended
Jul. 31, 2018
Jul. 31, 2019
Jan. 31, 2019
Sep. 30, 2017
Jun. 30, 2013
Convertible Debt [Abstract]          
Convertible Debt, Current   $ 1,233,189,000 $ 232,514,000    
Convertible Debt, Noncurrent   0 972,264,000    
2018 Notes          
Convertible Debt [Abstract]          
Principal         $ 350,000,000
Equity issuance costs $ 2,000,000        
2020 Notes          
Convertible Debt [Abstract]          
Principal   249,948,000 249,975,000   $ 250,000,000
Unamortized debt discount   (10,987,000) (16,480,000)    
Unamortized debt issuance costs   (644,000) (981,000)    
Convertible Debt, Current   238,317,000 232,514,000    
Carrying amount of the equity component   66,007,000 66,007,000    
Equity issuance costs   2,000,000      
2022 Notes          
Convertible Debt [Abstract]          
Principal   1,150,000,000 1,150,000,000 $ 1,150,000,000  
Unamortized debt discount   (146,070,000) (167,249,000)    
Unamortized debt issuance costs   (9,058,000) (10,487,000)    
Convertible Debt, Noncurrent   994,872,000 972,264,000    
Carrying amount of the equity component   219,702,000 $ 219,702,000    
Equity issuance costs   $ 4,000,000