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Fair Value Measurements - Narrative (Details)
Apr. 30, 2019
USD ($)
security
Sep. 30, 2018
USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Number of available-for-sale securities | security 155  
Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value of debt $ 615,400,000  
Convertible Debt | Convertible Senior Notes Due 2023    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt interest rate percentage 0.50%  
Aggregate principal amount of debt issued $ 575,000,000.0 $ 575,000,000.0