Basis Of Presentation And Significant Accounting Policies (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Basis Of Presentation And Significant Accounting Policies [Line Items]
Customer funds and cash and cash equivalents on deposit for regulatory requirements		$ 222	$ 182
Premiums receivable, allowance for doubtful accounts		278	302
Self-funded receivables, allowance for doubtful accounts		47	153
Other receivables, allowance for doubtful accounts		$ 280	306
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		95.00%
Advertising and marketing expense		$ 385	338	$ 246
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		11,300
Patient Protection Affordable Care Act Health Insurance Provider Fee, amount paid		$ 1,544		$ 1,176
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets (ASU 2016-01)					$ 91
Approximate percentage of revenues subject to new revenue guidance under ASC 606		6.00%
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income (ASU 2016-09)			$ 36
Cumulative Effect of New Accounting Principle in Period of Adoption (ASU 2018-02)					$ 320
Adoption of ASU 2016-09, tax benefit, amount per share			$ 0.13
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
Computer Equipment and Software [Member] | Minimum [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Property and equipment, useful life		3 years
Computer Equipment and Software [Member] | Maximum [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Property and equipment, useful life		5 years
Furniture And Other Equipment [Member] | Maximum [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Property and equipment, useful life		7 years
Software Development [Member] | Maximum [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Property and equipment, useful life		5 years
Subsequent Event [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Approximate effect of adoption of ASU 2016-02 on consolidated balance sheet	$ 700