Income Taxes (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Contingency [Line Items]
Income tax expense due to non-tax deductibility of the Health Insurance Provider fee payments	$ 411.7	$ 422.6	$ 312.6
Total deferred tax assets	1,304.0	1,343.5
Unrecognized tax benefits that would impact effective tax rate in future periods, if recognized	102.4
Unrecognized tax benefits that would impact additional paid-in capital in future periods, if recognized	2.4
Interest accrued	18.9	12.3
Net interest expense (benefit)	6.6	(1.8)	(4.2)
Unrecognized tax benefit change reasonably possible due to tax settlements - upper amount	3.1
Unrecognized tax benefit change reasonably possible due to tax settlements - lower amount	(75.1)
Federal tax net operating loss carry forwards	19.0
Income Taxes Paid	1,665.2	1,952.1	1,659.0
Health Insurance Provider fee [Member]
Income Tax Contingency [Line Items]
Income tax expense due to non-tax deductibility of the Health Insurance Provider fee payments	$ 411.7	$ 422.6	$ 312.6
Income tax expense per diluted share	$ 1.54	$ 1.55	$ 1.09
Adverse California Franchise Tax Ruling [Member]
Income Tax Contingency [Line Items]
Income tax expense as a result of an adverse franchise tax ruling		$ 42.3
Income tax expense per diluted share		$ 0.16
Latest Tax Year [Member]
Income Tax Contingency [Line Items]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2034
Earliest Tax Year [Member]
Income Tax Contingency [Line Items]
Operating Loss Carryforwards, Expiration Date	Jan. 01, 2017