Medical Claims Payable (Narrative) (Details) $ in Millions	Sep. 30, 2020 USD ($)
Short-duration Insurance Contracts, Accident Year 2018 and Prior Member] | Commercial Specialty Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 30
Short-duration Insurance Contracts, Accident Year 2018 and Prior Member] | Government Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	42
Short-duration Insurance Contracts, Accident Year 2018 and Prior Member] | Other Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	0
Short-Duration Insurance Contract, Accident Year 2019 [Member] | Commercial Specialty Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	189
Short-Duration Insurance Contract, Accident Year 2019 [Member] | Government Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	163
Short-Duration Insurance Contract, Accident Year 2019 [Member] | Other Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	0
Short-duration Insurance Contract, Accident Year 2020 [Member] | Commercial Specialty Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	2,646
Short-duration Insurance Contract, Accident Year 2020 [Member] | Government Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	6,596
Short-duration Insurance Contract, Accident Year 2020 [Member] | Other Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 196