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Significant Accounting Policies - Narrative (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2021
Dec. 31, 2020
Mar. 31, 2020
Schedule of Premium Revenue by Health Plan Type [Line Items]      
Risk adjustment payable $ 520 $ 326  
Risk adjustment receivable 20 20  
Risk adjustment, net payable 500 $ 306  
Risk adjustment, net payable, current year $ 211    
Risk adjustment, net payable, prior year     $ 289
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 $ 110