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Fair Value Measurements - Summary of Carrying Amounts and Estimated Fair Values of Financial Instruments (Detail) - USD ($)
$ in Thousands
Apr. 30, 2019
Jan. 31, 2019
Sep. 30, 2017
Jun. 30, 2013
1.50% Convertible senior notes        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Net Carrying Amount $ 235,408 $ 232,514    
Estimated Fair Value $ 590,566 557,074    
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 $ 983,507 972,264    
Estimated Fair Value $ 1,707,750 $ 1,560,228    
Contractual interest rate 0.25%   0.25%