Significant Accounting Policies - Due to Government Agencies (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Medicaid program:
Medical cost floors and corridors	$ 74	$ 103
Other amounts due to states	84	81
Marketplace program:
Risk adjustment	368	466
Cost sharing reduction	0	183
Other	138	134
Total	$ 664	$ 967