Medical Claims Payable (Narrative) (Details) $ in Millions	Mar. 31, 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	$ 94
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	63
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	763
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	885
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	2,182
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	5,453
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	$ 190