Medical Claims Payable (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-duration Insurance Contracts, Accident Year [Line Items]
Net incurred medical claims in prior years redundancies	$ (850.4)	$ (800.2)	$ (541.9)
Commercial and Specialty Business Segment [Member]
Short-duration Insurance Contracts, Accident Year [Line Items]
Net incurred medical claims in prior years redundancies	(466.5)	(480.3)	(239.5)
Commercial and Specialty Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2014 and Prior [Member]
Short-duration Insurance Contracts, Accident Year [Line Items]
Short-duration Insurance Contracts Incurred But Not Reported IBNR Claims Liability, Net	$ 26.2
Short-duration Insurance Contracts number of reported claims	131.6
Commercial and Specialty Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2015 [Member]
Short-duration Insurance Contracts, Accident Year [Line Items]
Short-duration Insurance Contracts Incurred But Not Reported IBNR Claims Liability, Net	$ 41.5
Short-duration Insurance Contracts number of reported claims	118.6
Commercial and Specialty Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2016 [Member]
Short-duration Insurance Contracts, Accident Year [Line Items]
Short-duration Insurance Contracts Incurred But Not Reported IBNR Claims Liability, Net	$ 2,678.0
Short-duration Insurance Contracts number of reported claims	111.2
Government Business Segment [Member]
Short-duration Insurance Contracts, Accident Year [Line Items]
Net incurred medical claims in prior years redundancies	$ (383.9)	$ (319.9)	$ (302.4)
Government Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2014 and Prior [Member]
Short-duration Insurance Contracts, Accident Year [Line Items]
Short-duration Insurance Contracts Incurred But Not Reported IBNR Claims Liability, Net	$ 5.5
Short-duration Insurance Contracts number of reported claims	193.0
Government Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2015 [Member]
Short-duration Insurance Contracts, Accident Year [Line Items]
Short-duration Insurance Contracts Incurred But Not Reported IBNR Claims Liability, Net	$ 65.4
Short-duration Insurance Contracts number of reported claims	189.0
Government Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2016 [Member]
Short-duration Insurance Contracts, Accident Year [Line Items]
Short-duration Insurance Contracts Incurred But Not Reported IBNR Claims Liability, Net	$ 4,536.9
Short-duration Insurance Contracts number of reported claims	193.6