Medical Claims Payable Medical Claims Payable (Reconciliation of the Claims Development to the Claims Liability) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net medical claims payable, end of period	$ 8,506	$ 7,232		$ 7,709
Ceded medical claims payable end of period	43	34	$ 35	105
Insurance lines other than short duration	209
Gross medical claims payable, end of period	8,758	7,454
Commercial Specialty Business Segment [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net medical claims payable, end of period	2,845	2,576		3,305
Ceded medical claims payable end of period	14	10	4	78
Insurance lines other than short duration	0
Gross medical claims payable, end of period	2,859
Government Business Segment [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net medical claims payable, end of period	5,661	4,656		4,404
Ceded medical claims payable end of period	29	$ 24	$ 31	$ 27
Insurance lines other than short duration	209
Gross medical claims payable, end of period	$ 5,899