Medical Claims Payable (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Claims Development [Line Items]
Net incurred medical claims in prior years redundancies	$ (1,164.6)	$ (850.4)	$ (800.2)
Commercial and Specialty Business Segment [Member]
Claims Development [Line Items]
Net incurred medical claims in prior years redundancies	(462.7)	(466.5)	(480.3)
Government Business Segment [Member]
Claims Development [Line Items]
Net incurred medical claims in prior years redundancies	(701.9)	$ (383.9)	$ (319.9)
Short-duration Insurance Contracts, Claim Year 2015 and Prior [Member] | Commercial and Specialty Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 34.1
Short-duration Insurance Contracts, Number of Reported Claims	124.5
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	$ 22.3
Short-duration Insurance Contracts, Number of Reported Claims	198.9
Short-duration Insurance Contracts, Claim Year 2016 [Member] | Commercial and Specialty Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 54.2
Short-duration Insurance Contracts, Number of Reported Claims	115.5
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	$ 77.6
Short-duration Insurance Contracts, Number of Reported Claims	204.0
Short-duration Insurance Contracts, Claim Year 2017 [Member] | Commercial and Specialty Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 3,240.3
Short-duration Insurance Contracts, Number of Reported Claims	113.1
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	$ 4,458.1
Short-duration Insurance Contracts, Number of Reported Claims	202.8