Medical Claims Payable (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Short-duration insurance contracts, claim year [Line Items]
Quality Improvement And Other Claims Expense	$ 368.9	$ 313.6
Benefit expense	17,542.8	$ 15,538.8
Commercial Specialty Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2015 and Prior [Member]
Short-duration insurance contracts, claim year [Line Items]
Short-duration Insurance Contracts Incurred But Not Reported IBNR Claims Liability, Net	93.1
Commercial Specialty Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2016 [Member]
Short-duration insurance contracts, claim year [Line Items]
Short-duration Insurance Contracts Incurred But Not Reported IBNR Claims Liability, Net	255.9
Commercial Specialty Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2017 [Member]
Short-duration insurance contracts, claim year [Line Items]
Short-duration Insurance Contracts Incurred But Not Reported IBNR Claims Liability, Net	2,524.1
Government Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2015 and Prior [Member]
Short-duration insurance contracts, claim year [Line Items]
Short-duration Insurance Contracts Incurred But Not Reported IBNR Claims Liability, Net	55.6
Government Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2016 [Member]
Short-duration insurance contracts, claim year [Line Items]
Short-duration Insurance Contracts Incurred But Not Reported IBNR Claims Liability, Net	865.2
Government Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2017 [Member]
Short-duration insurance contracts, claim year [Line Items]
Short-duration Insurance Contracts Incurred But Not Reported IBNR Claims Liability, Net	$ 3,672.4