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Significant Accounting Policies - Quality Incentive Premium Revenue (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Accounting Policies [Abstract]      
Maximum available quality incentive premium - current period $ 182 $ 150 $ 147
Amount of quality incentive premium revenue recognized in current period - Earned current period 133 97 104
Amount of quality incentive premium revenue recognized in current period - Earned prior periods 31 10 47
Total quality incentive premium revenue recognized $ 164 $ 107 $ 151
Quality incentive premium revenue recognized as a percentage of total premium revenue 0.90% 0.60% 0.90%