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Significant Accounting Policies - Quality Incentive Premium Revenue (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accounting Policies [Abstract]      
Maximum available quality incentive premium - current period $ 150 $ 147 $ 118
Amount of quality incentive premium revenue recognized in current period - Earned current period 97 104 66
Amount of quality incentive premium revenue recognized in current period - Earned prior periods 10 47 13
Total quality incentive premium revenue recognized $ 107 $ 151 $ 79
Quality incentive premium revenue recognized as a percentage of total premium revenue 0.60% 0.90% 0.60%