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Convertible Senior Notes, Net (Detail)
$ / shares in Units, shares in Millions
 3 Months Ended
Apr. 30, 2018
USD ($)
day
trading_day
$ / shares
shares
Apr. 30, 2017
USD ($)
Jan. 31, 2018
USD ($)
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 $ 2,000,000 $ 1,000,000      
2018 Notes          
Debt Instrument [Line Items]          
Contractual interest rate 0.75%       0.75%
Convertible senior notes, principal amount $ 350,000,000   $ 350,000,000   $ 350,000,000
Liability issuance costs 7,000,000        
Equity issuance costs 2,000,000        
Amortization expense for liability issuance costs $ 352,000 352,000      
Remaining life of the Notes 2 months        
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 $ 250,000,000   $ 250,000,000   $ 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 $ 168,000 168,000      
Remaining life of the Notes 26 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 $ 0      
Remaining life of the Notes 53 months        
Effective interest rates of the liability components 4.60%        
Aggregate amount for purchased options $ 176,000,000        
Proceeds from sale of warrants $ 81,000,000        
Shares related to the convertible senior notes          
Debt Instrument [Line Items]          
Repurchase of notes percentage 100.00%        
Warrants expires in October 2018          
Debt Instrument [Line Items]          
Shares covered by each purchased options or warrants (in shares) | shares 4.2        
Number of trading days over 60 days        
Exercise price of warrants (in dollars per share) | $ / shares $ 107.96        
Warrants expires in October 2020          
Debt Instrument [Line Items]          
Shares covered by each purchased options or warrants (in shares) | shares 3.1        
Number of trading days over 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 7.8        
Number of trading days over 60 days        
Exercise price of warrants (in dollars per share) | $ / shares $ 213.96        
2018 and 2020 Notes          
Debt Instrument [Line Items]          
Aggregate amount for purchased options $ 144,000,000        
Proceeds from sale of warrants $ 93,000,000        
Class A | 2018 Notes          
Debt Instrument [Line Items]          
Initial conversion rate 12.0075        
Principal amount converted into Class A Common Stock $ 1,000        
Initial conversion price (in dollars per share) | $ / shares $ 83.28        
Debt conversion, shares issued | shares 4.2        
Class A | 2020 Notes          
Debt Instrument [Line Items]          
Initial conversion rate 12.2340        
Principal amount converted into Class A Common Stock $ 1,000        
Initial conversion price (in dollars per share) | $ / shares $ 81.74        
Debt conversion, shares issued | shares 3.1        
Class A | 2022 Notes          
Debt Instrument [Line Items]          
Initial conversion rate 6.7982        
Principal amount converted into Class A Common Stock $ 1,000        
Initial conversion price (in dollars per share) | $ / shares $ 147.10        
Debt conversion, shares issued | shares 7.8        
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%