Medical Claims Payable Medical Claims Payable (Reconciliation of the Claims Development to the Claims Liability) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net medical claims payable, end of period	$ 9,565	$ 8,614		$ 7,232
Ceded medical claims payable end of period	90	33	$ 43	34
Insurance lines other than short duration	223
Gross medical claims payable, end of period	9,878	8,842
Commercial Specialty Business Segment [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net medical claims payable, end of period	2,737	3,025		2,576
Ceded medical claims payable end of period	62	14	14	10
Insurance lines other than short duration	0
Gross medical claims payable, end of period	2,799
Government Business Segment [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net medical claims payable, end of period	6,628	5,589		4,656
Ceded medical claims payable end of period	28	19	29	24
Insurance lines other than short duration	223
Gross medical claims payable, end of period	6,879
Other Segment [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net medical claims payable, end of period	200	0		0
Ceded medical claims payable end of period	0	$ 0	$ 0	$ 0
Insurance lines other than short duration	0
Gross medical claims payable, end of period	$ 200