Significant Accounting Policies - Amounts Due To Government Agencies (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Medicaid:
Minimum MLR and profit sharing	$ 735	$ 513
Other	167	76
Medicare:
Risk adjustment and Part D risk sharing	75	45
Minimum MLR and profit sharing	105	62
Other	42	30
Marketplace:
Risk adjustment	520	326
Minimum MLR	47	37
Other	27	21
Magellan Complete Care acquisition opening balance	0	143
Medical Premium Liability Due To Agency	$ 1,718	$ 1,253