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Significant Accounting Policies - Amounts Due To Government Agencies (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Accounting Policies [Abstract]    
Medical cost floors and corridors $ 98 $ 103
Other amounts due to states 90 81
Risk adjustment 568 466
Cost sharing reduction 0 183
Other 176 134
Medical Premium Liability Due To Agency $ 932 $ 967