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Convertible Senior Notes, Net (Detail)
$ / shares in Units, shares in Millions
3 Months Ended 9 Months Ended
Oct. 31, 2018
USD ($)
$ / shares
shares
Oct. 31, 2017
USD ($)
Oct. 31, 2018
USD ($)
day
trading_day
$ / shares
shares
Oct. 31, 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 $ 3,000,000 $ 3,000,000 $ 7,000,000 $ 6,000,000      
Proceeds from sale of warrants $ 0 80,805,000 $ 0 80,805,000      
2018 Notes              
Debt Instrument [Line Items]              
Contractual interest rate 0.75%   0.75%       0.75%
Convertible senior notes, principal amount $ 0   $ 0   $ 350,000,000   $ 350,000,000
Repayments of debt 350,000,000            
Liability issuance costs   7,000,000   7,000,000      
Equity issuance costs   2,000,000 2,000,000        
Amortization expense for liability issuance costs $ 0 352,000 $ 641,000 1,056,000      
Effective interest rates of the liability components 5.75%   5.75%        
2020 Notes              
Debt Instrument [Line Items]              
Contractual interest rate 1.50%   1.50%       1.50%
Convertible senior notes, principal amount $ 249,997,000   $ 249,997,000   $ 250,000,000   $ 250,000,000
Threshold percentage of conversion price     130.00%        
Liability issuance costs 5,000,000   $ 5,000,000        
Equity issuance costs     2,000,000        
Amortization expense for liability issuance costs $ 167,000 168,000 $ 504,000 503,000      
Remaining life of the Notes     20 months        
Effective interest rates of the liability components 6.25%   6.25%        
2022 Notes              
Debt Instrument [Line Items]              
Contractual interest rate 0.25%   0.25%     0.25%  
Convertible senior notes, principal amount           $ 1,150,000,000.00  
Liability issuance costs $ 14,000,000   $ 14,000,000        
Equity issuance costs     4,000,000        
Amortization expense for liability issuance costs $ 714,000 $ 365,000 $ 2,143,000 $ 365,000      
Remaining life of the Notes     47 months        
Effective interest rates of the liability components 4.60%   4.60%        
Aggregate amount for purchased options $ 176,000,000   $ 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%        
2018 and 2020 Notes              
Debt Instrument [Line Items]              
Aggregate amount for purchased options 144,000,000   $ 144,000,000        
Proceeds from sale of warrants     93,000,000        
Warrants expires in October 2018              
Debt Instrument [Line Items]              
Aggregate amount for purchased options $ 3,500,000   $ 3,500,000        
Shares covered by each purchased options or warrants (in shares) | shares 4.2   4.2        
Number of trading days over     60 days        
Exercise price of warrants (in dollars per share) | $ / shares $ 107.96   $ 107.96        
Warrants expires in October 2020              
Debt Instrument [Line Items]              
Shares covered by each purchased options or warrants (in shares) | shares 3.1   3.1        
Number of trading days over     60 days        
Exercise price of warrants (in dollars per share) | $ / shares $ 107.96   $ 107.96        
Warrants Expires In January 2023              
Debt Instrument [Line Items]              
Shares covered by each purchased options or warrants (in shares) | shares 7.8   7.8        
Number of trading days over     60 days        
Exercise price of warrants (in dollars per share) | $ / shares $ 213.96   $ 213.96        
Class A | 2018 Notes              
Debt Instrument [Line Items]              
Shares issued (in shares) | shares 1.5            
Initial conversion price (in dollars per share) | $ / shares $ 83.28   $ 83.28        
Debt conversion, shares issued | shares 1.5   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   $ 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   $ 147.10        
Debt conversion, shares issued | shares     7.8        
Class A | Warrants expires in October 2018              
Debt Instrument [Line Items]              
Shares issued (in shares) | shares 0.1            
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%