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Significant Accounting Policies - Narrative (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2021
Dec. 31, 2020
Sep. 30, 2020
Schedule of Premium Revenue by Health Plan Type [Line Items]        
Risk adjustment payable $ 691 $ 691 $ 326  
Risk adjustment receivable 29 29 20  
Risk adjustment, net payable 662 662 $ 306  
Risk adjustment, net payable, current year (660) $ (660)    
Risk adjustment, net payable, prior year       $ (2)
Maturity period (less than)   10 years    
Structured Securities        
Schedule of Premium Revenue by Health Plan Type [Line Items]        
Average maturity period (less than)   10 years    
COVID-19        
Schedule of Premium Revenue by Health Plan Type [Line Items]        
Reduction in premiums $ 17 $ 183