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Convertible Senior Notes, Net (Detail)
3 Months Ended 6 Months Ended 12 Months Ended
Jul. 31, 2018
USD ($)
shares
Jan. 31, 2019
USD ($)
$ / shares
shares
Jan. 31, 2019
USD ($)
day
trading_day
$ / shares
shares
Jan. 31, 2018
USD ($)
Jan. 31, 2017
USD ($)
Sep. 30, 2017
USD ($)
Jun. 30, 2013
USD ($)
Debt Instrument [Line Items]              
Purchase of convertible senior notes hedges     $ 0 $ 175,530,000 $ 0    
Threshold consecutive trading days | trading_day     30        
Number of days out of thirty that common stock price exceeded conversion price | day     20        
Interest costs capitalized     $ 11,000,000 8,000,000 3,000,000    
Proceeds from sale of warrants     $ 0 80,805,000 0    
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     $ 350,000,000.0
Repayments of debt $ 350,000,000            
Debt issuance costs   $ 7,000,000 7,000,000        
Equity issuance costs     2,000,000        
Amortization expense for debt issuance costs     $ 641,000 1,409,000 1,408,000    
Effective interest rates of the liability components, percentage   5.75% 5.75%        
2018 Notes | Class A Common Stock              
Debt Instrument [Line Items]              
Shares issued (in shares) | shares 1,500,000            
Initial conversion price (in dollars per share) | $ / shares   $ 83.28 $ 83.28        
Settlement of convertible senior notes (in shares) | shares 1,500,000            
Indexed shares (in shares) | shares   4,200,000 4,200,000        
2020 Notes              
Debt Instrument [Line Items]              
Contractual interest rate   1.50% 1.50%       1.50%
Convertible Senior Notes, principal amount   $ 249,975,000 $ 249,975,000 250,000,000     $ 250,000,000.0
Threshold percentage of conversion price     130.00%        
Debt issuance costs   $ 5,000,000 $ 5,000,000        
Equity issuance costs     2,000,000        
Amortization expense for debt issuance costs     $ 673,000 673,000 675,000    
Remaining life of the notes, months     17 months        
Effective interest rates of the liability components, percentage   6.25% 6.25%        
2020 Notes | Class A Common Stock              
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        
Indexed shares (in shares) | shares   3,100,000 3,100,000        
2022 Notes              
Debt Instrument [Line Items]              
Contractual interest rate   0.25% 0.25%     0.25%  
Convertible Senior Notes, principal amount   $ 1,150,000,000 $ 1,150,000,000 1,150,000,000   $ 1,150,000,000.00  
Purchase of convertible senior notes hedges     176,000,000        
Debt issuance costs   $ 14,000,000 14,000,000        
Equity issuance costs     4,000,000        
Amortization expense for debt issuance costs     $ 2,858,000 $ 1,080,000 $ 0    
Remaining life of the notes, months     44 months        
Effective interest rates of the liability components, percentage   4.60% 4.60%        
Proceeds from sale of warrants     $ 81,000,000        
2022 Notes | Class A Common Stock              
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        
Indexed shares (in shares) | shares   7,800,000 7,800,000        
Convertible Senior Notes              
Debt Instrument [Line Items]              
Repurchase of notes percentage     100.00%        
Convertible Senior Notes | Debt Conversion, Option One              
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%        
Convertible Senior Notes | Debt Conversion, Option Two              
Debt Instrument [Line Items]              
Threshold consecutive trading days | trading_day     5        
Threshold business days | trading_day     5        
Convertible Senior Notes | Debt Conversion, Option Two | Maximum              
Debt Instrument [Line Items]              
Threshold percentage of conversion price     98.00%        
FaceValuePerNote              
Debt Instrument [Line Items]              
Convertible Senior Notes, principal amount   $ 1,000 $ 1,000        
Notes 2018 and 2020              
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 option/warrant, shares | shares   4,200,000 4,200,000        
Trading days     60 days        
Exercise price of warrants, dollars per share | $ / shares   $ 107.96 $ 107.96        
Warrants Expires In October 2020              
Debt Instrument [Line Items]              
Shares covered by each purchased option/warrant, shares | shares   3,100,000 3,100,000        
Trading days     60 days        
Exercise price of warrants, dollars per share | $ / shares   $ 107.96 $ 107.96        
Warrants Expires In January 2023              
Debt Instrument [Line Items]              
Shares covered by each purchased option/warrant, shares | shares   7,800,000 7,800,000        
Trading days     60 days        
Exercise price of warrants, dollars per share | $ / shares   $ 213.96 $ 213.96        
Warrants Expires in July 2018              
Debt Instrument [Line Items]              
Aggregate amount paid for Purchased Options   $ 0 $ 0        
Warrants Expires in July 2018 | Class A Common Stock              
Debt Instrument [Line Items]              
Shares issued (in shares) | shares   1,100,000