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Significant Accounting Policies - Quality Incentive Premium Revenue (Details) - State health plans with quality incentive programs - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Schedule of Premium Revenue by Health Plan Type [Line Items]      
Maximum available quality incentive premium - current period $ 147 $ 118 $ 90
Amount of quality incentive premium revenue recognized in current period - Earned current period 104 66 40
Amount of quality incentive premium revenue recognized in current period - Earned prior periods 47 13 4
Total quality incentive premium revenue recognized $ 151 $ 79 $ 44
Quality incentive premium revenue recognized as a percentage of total premium revenue 0.90% 0.60% 0.50%