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Significant Accounting Policies - Quality Incentive Premium Revenue Recognized (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Schedule of Premium Revenue by Health Plan Type [Line Items]        
Maximum available quality incentive premium - current period $ 65 $ 46 $ 126 $ 91
Quality incentive premium revenue recognized in current period:        
Earned current period 58 37 102 63
Earned prior periods 7 10 19 30
Total $ 65 $ 47 $ 121 $ 93
Quality incentive premium revenue recognized as a percentage of total premium revenue 1.50% 1.20% 1.40% 1.20%
Select health plans | Minimum        
Schedule of Premium Revenue by Health Plan Type [Line Items]        
Percentage of additional incremental revenue earned     1.00%  
Select health plans | Maximum        
Schedule of Premium Revenue by Health Plan Type [Line Items]        
Percentage of additional incremental revenue earned     4.00%