Basis Of Presentation And Significant Accounting Policies (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2020	Jan. 01, 2019	Jan. 01, 2018
Basis Of Presentation And Significant Accounting Policies [Line Items]
Customer funds and cash and cash equivalents on deposit for regulatory requirements	$ 215	$ 222
Premiums receivable, allowance for doubtful accounts	237	278
Self-funded receivables, allowance for doubtful accounts	46	47
Other receivables, allowance for doubtful accounts	$ 242	280
Securities Lending Transactions Initial Collateral Percentage Value	102.00%
Reserves for future policy benefits, maximum payable period	1 year
Defined Benefit Plan, Alternative Method Used to Amortize Gain (Loss), Description	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.
Employee stock purchase plan, purchase price per share as a percent of closing price	90.00%
Advertising and marketing expense	$ 467	385	$ 338
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		14,300
Patient Protection Affordable Care Act Health Insurance Provider Fee, amount paid		1,544
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets (ASU 2018-02)						$ 91
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income (ASU 2016-09)			$ 36
Adoption of ASU 2016-09, tax benefit, amount per share			$ 0.13
Cumulative effect of new accounting principle in period of adoption		$ 26				$ 320
Accounting Standards Update 2016-02 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Operating Lease, Liability					$ 728
Operating Lease, Right-of-Use Asset					637
Cumulative effect of new accounting principle in period of adoption					$ 26
Subsequent Event [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
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				$ 15,523
Minimum [Member] | Buildings and Improvements [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life	15 years
Minimum [Member] | Computer Equipment and Software [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life	3 years
Minimum [Member] | Software Development [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Maximum [Member] | Buildings and Improvements [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life	39 years
Maximum [Member] | Computer Equipment and Software [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Maximum [Member] | Furniture And Other Equipment [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life	7 years
Maximum [Member] | Software Development [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life	10 years