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Significant Accounting Policies - Due to Government Agencies (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Medicaid program:    
Medical cost floors and corridors $ 103 $ 135
Other amounts due to states 81 71
Marketplace program:    
Risk adjustment 466 917
Cost sharing reduction 183 275
Other 134 144
Amounts due government agencies $ 967 $ 1,542