Basis Of Presentation And Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Basis Of Presentation And Significant Accounting Policies [Line Items]
Defined Benefit Plan, Alternative Method Used to Amortize Net Gains and Losses	We determine the expected return on plan assets using the calculated value of plan assets, which recognizes changes in the fair value of plan assets in a systematic manner over three years. We apply a corridor approach to amortize unrecognized actuarial gains or losses. Under this approach, only accumulated net actuarial gains or losses in excess of 10% of the greater of the projected benefit obligation or the fair value of plan assets are amortized over the average remaining service or lifetime of the workforce as a component of net periodic benefit cost.
Securities lending transactions, initial percentage value of collateral required to value of securities borrowed	102.00%
Securities lending transactions, fair value of collateral received	$ 1,515.3	$ 969.7
Securities lending transactions, minimum percentage of value of collateral required to be maintained by borrower in relation to the value of securities borrowed	100.00%
Securities lending transactions, ratio of fair value of collateral on hold to fair value of securities on loan	103.00%	102.00%
Allowance for doubtful accounts, premium and self-funded receivables	213.6	223.6
Allowance for doubtful accounts, other receivables	142.2	115.0
Derivative liability, collateral, right to reclaim cash, offset	127.8
Derivative asset, collateral, obligation to return cash, offset	105.0
Reserves for future policy benefits, maximum payable period	1 year
Employee stock purchase plan, purchase price per share as a percent of closing price	95.00%
Advertising and marketing expense	337.0	350.9	285.4
Patient Protection Affordable Care Act Health Insurance Provider Fee, annual amount to be collected by the Government through assessments to all health insurers subject to the fee	8,000.0
Patient Protection Affordable Care Act Health Insurance Provider Fee, amount paid	$ 893.3
Buildings and Improvements [Member] | Minimum [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Property and equipment, useful life	15 years
Buildings and Improvements [Member] | Maximum [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Property and equipment, useful life	39 years
Data Processing Equipment, Furniture And Other Equipment [Member] | Minimum [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Property and equipment, useful life	3 years
Data Processing Equipment, Furniture And Other Equipment [Member] | Maximum [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Property and equipment, useful life	7 years
Computer Software [Member] | Minimum [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Property and equipment, useful life	3 years
Computer Software [Member] | Maximum [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Property and equipment, useful life	5 years