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Convertible Senior Notes, Net - Schedule of Senior Notes (Detail) - USD ($)
3 Months Ended
Apr. 30, 2018
Apr. 30, 2017
Jan. 31, 2018
Jun. 30, 2013
Convertible Debt [Abstract]        
Net carrying amount, current $ 570,251,000   $ 341,509,000  
Net carrying amount, non-current 939,242,000   1,149,845,000  
2018 Notes        
Debt Instrument [Line Items]        
Contractual interest expense 656,000 $ 656,000    
Convertible Debt [Abstract]        
Principal 350,000,000   350,000,000 $ 350,000,000
Unamortized debt discount (3,567,000)   (7,850,000)  
Unamortized debt issuance costs (289,000)   (641,000)  
Net carrying amount, current 346,144,000   341,509,000  
Carrying amount of the equity component 74,892,000   74,892,000  
Equity issuance costs 2,000,000      
2020 Notes        
Debt Instrument [Line Items]        
Contractual interest expense 937,000 $ 938,000    
Convertible Debt [Abstract]        
Principal 250,000,000   250,000,000 $ 250,000,000
Unamortized debt discount (24,407,000)   (26,968,000)  
Unamortized debt issuance costs (1,486,000)   (1,654,000)  
Net carrying amount, current 224,107,000      
Net carrying amount, non-current     221,378,000  
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  
Unamortized debt discount (198,127,000)   (208,188,000)  
Unamortized debt issuance costs (12,631,000)   (13,345,000)  
Net carrying amount, non-current 939,242,000   928,467,000  
Carrying amount of the equity component $ 219,702,000   $ 219,702,000