Basis Of Presentation And Significant Accounting Policies (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2020
Basis Of Presentation And Significant Accounting Policies [Line Items]
Customer funds and cash and cash equivalents on deposit for regulatory requirements	$ 170,000,000	$ 215,000,000
Securities Lending Transactions Initial Collateral Percentage Value	102.00%	103.00%
Premiums receivable, allowance for doubtful accounts	$ 146,000,000	$ 237,000,000
Self-funded receivables, allowance for doubtful accounts	54,000,000	46,000,000
Other receivables, allowance for doubtful accounts	$ 374,000,000	242,000,000
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.
Reserves for future policy benefits, maximum payable period	1 year
Employee stock purchase plan, purchase price per share as a percent of closing price	90.00%
Advertising and marketing expense	$ 558,000,000	467,000,000	$ 385,000,000
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	15,523,000,000		14,300,000,000
Patient Protection Affordable Care Act Health Insurance Provider Fee, amount paid	1,570,000,000		$ 1,544,000,000
Retained Earnings (Accumulated Deficit)	23,802,000,000	$ 22,573,000,000
Operating Lease, Impairment Loss	$ 258,000,000
Accounting Standards Update 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjustment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Retained Earnings (Accumulated Deficit)				$ (35,000,000)
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] | Internal-use software [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] | Internal-use software [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life	10 years