Medical Claims and Benefits Payable - Reconciliation of Claims Development (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Insurance [Abstract]
Incurred claims and allocated claims adjustment expenses	$ 47,263
Less: cumulative paid clams and allocated claims adjustment expenses	(45,462)
All outstanding liabilities before 2016	10
Non-risk provider payables and other	150
Medical claims and benefits payable	$ 1,961	$ 2,192	$ 1,929	$ 1,685