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Fair Value Measurement - Narrative (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Sep. 30, 2017
Aug. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Equity securities without readily determinable value, fair value $ 4.5    
Convertible Debt | 2021 Notes      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Interest rate (as a percent)     1.875%
Convertible Debt | 2024 Notes      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Interest rate (as a percent)   1.50%  
Significant Other Observable Inputs (Level 2) | Convertible Debt | 2021 Notes      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Fair value of exchangeable senior notes 558.0    
Significant Other Observable Inputs (Level 2) | Convertible Debt | 2024 Notes      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Fair value of exchangeable senior notes $ 521.0