Medical Claims Payable (Narrative) (Details) $ in Millions	Sep. 30, 2017 USD ($)
Short-duration Insurance Contracts, Claim Year 2017 [Member] | Commercial Specialty Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 3,147.3
Short-duration Insurance Contracts, Claim Year 2017 [Member] | Government Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	4,360.1
Short-duration Insurance Contracts, Claim Year 2016 [Member] | Commercial Specialty Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	141.6
Short-duration Insurance Contracts, Claim Year 2016 [Member] | Government Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	136.9
Short-duration Insurance Contracts, Claim Year 2015 and Prior [Member] | Commercial Specialty Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	56.4
Short-duration Insurance Contracts, Claim Year 2015 and Prior [Member] | Government Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 20.2