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Fair Value Measurements - Summary of Carrying Amounts and Estimated Fair Values of Financial Instruments (Detail) - USD ($)
$ in Thousands
Apr. 30, 2018
Jan. 31, 2018
Sep. 30, 2017
Jun. 30, 2013
0.75% Convertible senior notes        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Net Carrying Amount $ 346,144 $ 341,509    
Estimated Fair Value $ 520,748 504,994    
Contractual interest rate 0.75%     0.75%
1.50% Convertible senior notes        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Net Carrying Amount $ 224,107 221,378    
Estimated Fair Value $ 392,463 385,550    
Contractual interest rate 1.50%     1.50%
0.25% Convertible senior notes        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Net Carrying Amount $ 939,242 928,467    
Estimated Fair Value $ 1,247,865 $ 1,200,577    
Contractual interest rate 0.25%   0.25%