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Significant Accounting Policies - Quality Incentive Premium Revenue Recognized (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Schedule of Premium Revenue by Health Plan Type [Line Items]        
Maximum available quality incentive premium - current period $ 36 $ 33 $ 113 $ 114
Quality incentive premium revenue recognized in current period:        
Earned current period 24 26 72 80
Earned prior periods 3 0 9 54
Total $ 27 $ 26 $ 81 $ 134
Quality incentive premium revenue recognized as a percentage of total premium revenue 0.60% 0.60% 0.60% 1.10%
Minimum | Select Health Plans        
Schedule of Premium Revenue by Health Plan Type [Line Items]        
Percentage of additional incremental revenue earned     1.00%  
Maximum | Select Health Plans        
Schedule of Premium Revenue by Health Plan Type [Line Items]        
Percentage of additional incremental revenue earned     3.00%