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Fair Value Measurement - Narrative (Details) - USD ($)
$ in Millions
Sep. 30, 2020
Jun. 30, 2020
Feb. 15, 2018
Feb. 15, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Equity securities without readily determinable fair value $ 4.5      
2021 Notes | Convertible Debt        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Interest rate (as a percent)       1.875%
2024 Notes | Convertible Debt        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Interest rate (as a percent)     1.50%  
2026 Notes | Convertible Debt        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Interest rate (as a percent)   2.00%    
Significant Other Observable Inputs (Level 2) | 2021 Notes | Convertible Debt        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Fair value of exchangeable senior notes 222.0      
Significant Other Observable Inputs (Level 2) | 2024 Notes | Convertible Debt        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Fair value of exchangeable senior notes 584.0      
Significant Other Observable Inputs (Level 2) | 2026 Notes | Convertible Debt        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Fair value of exchangeable senior notes $ 1,200.0