Medical Claims Payable Medical Claims Payable (Reconciliation of the Claims Development to the Claims Liability) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net medical claims payable, end of period	$ 9,862	$ 8,614		$ 7,232
Ceded medical claims payable end of period	129	33	$ 35	34
Insurance lines other than short duration	261
Gross medical claims payable, end of period	10,252	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	2,865	3,025		2,576
Ceded medical claims payable end of period	102	14	8	10
Insurance lines other than short duration	0
Gross medical claims payable, end of period	2,967
Government Business Segment [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net medical claims payable, end of period	6,801	5,589		4,656
Ceded medical claims payable end of period	27	19	27	24
Insurance lines other than short duration	261
Gross medical claims payable, end of period	7,089
Other Segment [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net medical claims payable, end of period	196	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	$ 196