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Convertible Senior Notes, Net - Schedule of Senior Notes (Detail) - USD ($)
3 Months Ended
Apr. 30, 2019
Apr. 30, 2018
Jan. 31, 2019
Jun. 30, 2013
Convertible Debt [Abstract]        
Convertible Debt, Current $ 235,408,000   $ 232,514,000  
Convertible Debt, Noncurrent 983,507,000   972,264,000  
2018 Notes        
Convertible Debt [Abstract]        
Principal       $ 350,000,000
Equity issuance costs   $ 2,000,000    
2020 Notes        
Convertible Debt [Abstract]        
Principal 249,975,000   249,975,000 $ 250,000,000
Unamortized debt discount (13,755,000)   (16,480,000)  
Unamortized debt issuance costs (812,000)   (981,000)  
Convertible Debt, Current 235,408,000   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  
Unamortized debt discount (156,720,000)   (167,249,000)  
Unamortized debt issuance costs (9,773,000)   (10,487,000)  
Convertible Debt, Noncurrent 983,507,000   972,264,000  
Carrying amount of the equity component $ 219,702,000   $ 219,702,000