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Significant Accounting Policies - Convertible Senior Notes (Details) - Senior Notes - USD ($)
1 Months Ended
Aug. 31, 2020
Apr. 30, 2019
Apr. 30, 2018
Dec. 31, 2020
Dec. 31, 2019
Mar. 31, 2019
0% Convertible Senior Notes Due 2026            
Debt Instrument [Line Items]            
Face amount $ 1,000,000,000.0     $ 1,000,000,000    
Interest rate, stated percentage 0.00%          
Principal amount $ 1,000,000,000          
0.125 Percent Convertible Senior Notes Due 2025            
Debt Instrument [Line Items]            
Face amount       800,000,000 $ 800,000,000 $ 700,000,000
Interest rate, stated percentage           0.125%
Option to purchase additional notes   $ 100,000,000        
Principal amount   $ 800,000,000        
0.25% Convertible Senior Notes Due 2023            
Debt Instrument [Line Items]            
Face amount     $ 345,000,000 $ 115,576,000 $ 345,000,000  
Interest rate, stated percentage     0.25%      
Option to purchase additional notes     $ 45,000,000      
Principal amount     $ 345,000,000