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Convertible Senior Notes, Net (Detail)
3 Months Ended 6 Months Ended
Jul. 31, 2019
USD ($)
$ / shares
shares
Jul. 31, 2019
USD ($)
$ / shares
shares
Apr. 30, 2019
trading_day
Jul. 31, 2018
USD ($)
Jul. 31, 2019
USD ($)
trading_day
$ / shares
shares
Jul. 31, 2018
USD ($)
Jan. 31, 2019
USD ($)
Sep. 30, 2017
USD ($)
Jun. 30, 2013
USD ($)
Debt Instrument [Line Items]                  
Threshold consecutive trading days | trading_day     30   30        
Number of days out of 30 that common stock price exceeded conversion price (in days) | trading_day     20   20        
Capitalized interest costs   $ 2,000,000   $ 2,000,000 $ 6,000,000 $ 4,000,000      
2018 Notes                  
Debt Instrument [Line Items]                  
Contractual interest rate                 0.75%
Convertible senior notes, principal amount                 $ 350,000,000
Repayments of debt   350,000,000              
Liability issuance costs       7,000,000   7,000,000      
Equity issuance costs       2,000,000          
Amortization expense for liability issuance costs   $ 0   289,000 $ 0 641,000      
Effective interest rates of the liability components 5.75% 5.75%     5.75%        
2020 Notes                  
Debt Instrument [Line Items]                  
Contractual interest rate 1.50% 1.50%     1.50%       1.50%
Convertible senior notes, principal amount $ 249,948,000 $ 249,948,000     $ 249,948,000   $ 249,975,000   $ 250,000,000
Threshold percentage of conversion price     130.00%   130.00%        
Liability issuance costs $ 5,000,000 5,000,000     $ 5,000,000        
Equity issuance costs         2,000,000        
Amortization expense for liability issuance costs   $ 168,000   169,000 $ 337,000 337,000      
Remaining life of the Notes 11 months                
Effective interest rates of the liability components 6.25% 6.25%     6.25%        
2022 Notes                  
Debt Instrument [Line Items]                  
Contractual interest rate 0.25% 0.25%     0.25%     0.25%  
Convertible senior notes, principal amount $ 1,150,000,000 $ 1,150,000,000     $ 1,150,000,000   $ 1,150,000,000 $ 1,150,000,000  
Liability issuance costs $ 14,000,000 14,000,000     14,000,000        
Equity issuance costs         4,000,000        
Amortization expense for liability issuance costs   $ 715,000   $ 715,000 $ 1,429,000 $ 1,429,000      
Remaining life of the Notes 38 months                
Effective interest rates of the liability components 4.60% 4.60%     4.60%        
Purchase of convertible senior notes hedges         $ 176,000,000        
Proceeds from sale of warrants         81,000,000        
Face value per note                  
Debt Instrument [Line Items]                  
Convertible senior notes, principal amount $ 1,000 $ 1,000     $ 1,000        
Shares related to the convertible senior notes                  
Debt Instrument [Line Items]                  
Repurchase of notes percentage         100.00%        
2018 and 2020 Notes                  
Debt Instrument [Line Items]                  
Purchase of convertible senior notes hedges         $ 144,000,000        
Proceeds from sale of warrants         $ 93,000,000        
Warrants expires in October 2018                  
Debt Instrument [Line Items]                  
Shares covered by each purchased options or warrants (in shares) | shares 4,200,000 4,200,000     4,200,000        
Number of trading days related to warrants (in days)         60 days        
Exercise price of warrants (in dollars per share) | $ / shares $ 107.96 $ 107.96     $ 107.96        
Warrants outstanding $ 0 $ 0     $ 0        
Warrants expires in October 2020                  
Debt Instrument [Line Items]                  
Shares covered by each purchased options or warrants (in shares) | shares 3,100,000 3,100,000     3,100,000        
Number of trading days related to warrants (in days)         60 days        
Exercise price of warrants (in dollars per share) | $ / shares $ 107.96 $ 107.96     $ 107.96        
Warrants Expires In January 2023                  
Debt Instrument [Line Items]                  
Shares covered by each purchased options or warrants (in shares) | shares 7,800,000 7,800,000     7,800,000        
Number of trading days related to warrants (in days)         60 days        
Exercise price of warrants (in dollars per share) | $ / shares $ 213.96 $ 213.96     $ 213.96        
Class A | 2018 Notes                  
Debt Instrument [Line Items]                  
Shares issued (in shares) | shares   1,500,000              
Initial conversion price (in dollars per share) | $ / shares $ 83.28 $ 83.28     $ 83.28        
Indexed shares (in shares) | shares 4,200,000 4,200,000     4,200,000        
Class A | 2020 Notes                  
Debt Instrument [Line Items]                  
Debt instrument, convertible, number of shares | shares         12.2340        
Initial conversion price (in dollars per share) | $ / shares $ 81.74 $ 81.74     $ 81.74        
Indexed shares (in shares) | shares 3,100,000 3,100,000     3,100,000        
Class A | 2022 Notes                  
Debt Instrument [Line Items]                  
Debt instrument, convertible, number of shares | shares         6.7982        
Initial conversion price (in dollars per share) | $ / shares $ 147.10 $ 147.10     $ 147.10        
Indexed shares (in shares) | shares 7,800,000 7,800,000     7,800,000        
Class A | Warrants expires in October 2018                  
Debt Instrument [Line Items]                  
Shares issued (in shares) | shares   1,100,000              
Debt Conversion, Option One | Shares related to the convertible senior notes                  
Debt Instrument [Line Items]                  
Threshold trading days (in trading days) | trading_day         20        
Threshold consecutive trading days | trading_day         30        
Threshold percentage of conversion price         130.00%        
Debt Conversion, Option Two | Shares related to the convertible senior notes                  
Debt Instrument [Line Items]                  
Threshold trading days (in trading days) | trading_day         5        
Debt Conversion, Option Two | Shares related to the convertible senior notes | Maximum                  
Debt Instrument [Line Items]                  
Threshold percentage of conversion price         98.00%