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Fair Value Measurements - Narrative (Detail) - USD ($)
3 Months Ended
Apr. 30, 2018
Jan. 31, 2018
Sep. 30, 2017
Jun. 30, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Closing price of company's common stock, dollars per share $ 124.84      
0.75% Convertible senior notes        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Convertible senior notes, principal amount $ 350,000,000 $ 350,000,000   $ 350,000,000
Contractual interest rate 0.75%     0.75%
If-converted value in excess of principal $ 175,000,000      
1.50% Convertible senior notes        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Convertible senior notes, principal amount $ 250,000,000 $ 250,000,000   $ 250,000,000
Contractual interest rate 1.50%     1.50%
If-converted value in excess of principal $ 132,000,000      
0.25% Convertible senior notes        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Convertible senior notes, principal amount     $ 1,150,000,000.00  
Contractual interest rate 0.25%   0.25%  
If-converted value less than principal $ 174,000,000