Medical Claims Payable Medical Claims Payable (Reconciliation of the Claims Development to the Claims Liability) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net medical claims payable, end of period	$ 9,630	$ 8,614		$ 7,232
Ceded medical claims payable end of period	60	33	$ 34	34
Insurance lines other than short duration	212
Gross medical claims payable, end of period	9,902	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	3,039	3,025		2,576
Ceded medical claims payable end of period	37	14	8	10
Insurance lines other than short duration	0
Gross medical claims payable, end of period	3,076
Government Business Segment [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net medical claims payable, end of period	6,401	5,589		4,656
Ceded medical claims payable end of period	23	19	26	24
Insurance lines other than short duration	212
Gross medical claims payable, end of period	6,636
Other Segment [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net medical claims payable, end of period	190	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	$ 190