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Convertible Senior Notes, Net (Detail)
3 Months Ended
Apr. 30, 2019
USD ($)
day
trading_day
$ / shares
shares
Jul. 31, 2018
USD ($)
shares
Apr. 30, 2018
USD ($)
Jan. 31, 2019
USD ($)
Sep. 30, 2017
USD ($)
Jun. 30, 2013
USD ($)
Debt Instrument [Line Items]            
Threshold consecutive trading days | trading_day 30          
Number of days out of 30 that common stock price exceeded conversion price (in days) | day 20          
Capitalized interest costs $ 4,000,000   $ 2,000,000      
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      
Equity issuance costs     2,000,000      
Amortization expense for liability issuance costs $ 0   352,000      
Effective interest rates of the liability components 5.75%          
2020 Notes            
Debt Instrument [Line Items]            
Contractual interest rate 1.50%         1.50%
Convertible senior notes, principal amount $ 249,975,000     $ 249,975,000   $ 250,000,000
Threshold percentage of conversion price 130.00%          
Liability issuance costs $ 5,000,000          
Equity issuance costs 2,000,000          
Amortization expense for liability issuance costs $ 169,000   168,000      
Remaining life of the Notes 14 months          
Effective interest rates of the liability components 6.25%          
2022 Notes            
Debt Instrument [Line Items]            
Contractual interest rate 0.25%       0.25%  
Convertible senior notes, principal amount         $ 1,150,000,000.00  
Liability issuance costs $ 14,000,000          
Equity issuance costs 4,000,000          
Amortization expense for liability issuance costs $ 714,000   $ 700,000      
Remaining life of the Notes 41 months          
Effective interest rates of the liability components 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          
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          
Number of trading days related to warrants (in days) 60 days          
Exercise price of warrants (in dollars per share) | $ / shares $ 107.96          
Warrants outstanding $ 0          
Warrants expires in October 2020            
Debt Instrument [Line Items]            
Shares covered by each purchased options or warrants (in shares) | shares 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          
Warrants Expires In January 2023            
Debt Instrument [Line Items]            
Shares covered by each purchased options or warrants (in shares) | shares 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          
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          
Indexed shares (in shares) | shares 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          
Indexed shares (in shares) | shares 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          
Indexed shares (in shares) | shares 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%