10-K Basis Of Presentation And Significant Accounting Policies (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Jan. 01, 2022	Dec. 31, 2019
Basis Of Presentation And Significant Accounting Policies [Line Items]
Customer funds and cash and cash equivalents on deposit for regulatory requirements	$ 300						$ 300		$ 258	$ 173
Securities lending transactions ratio of fair value of collateral held to fair value of securities loaned	102.00%						102.00%		102.00%	102.00%
Premium receivable, allowance for doubtful accounts	$ 188						$ 188		$ 152	$ 142
Self-funded receivables, allowance for doubtful accounts	93						93		68	50
Allowance for doubtful accounts, other receivables	875						875		$ 744	648
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									$ 511	588	$ 558
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
Patient Protection Affordable Care Act Health Insurance Provider Fee, amount paid											1,570
Operating Lease, Impairment Loss									34	136	258
Long-term Debt									23,849	22,756
Total Shareholders' Balance, beginning	38,423			$ 36,055			38,423	$ 36,055	36,243	36,095
Shareholders' net income	1,289			1,603			5,131	5,029	5,894	6,158	4,572
Benefit expense	30,606	$ 31,604	$ 30,786	29,421	$ 28,795	$ 28,231	92,996	86,447	116,642	102,571	88,045
Total assets	110,478						110,478		102,755	97,456
Total liabilities	71,939						71,939		66,425	61,293
Accumulated other comprehensive loss	(2,512)	(2,166)		(2,779)	(2,151)		(2,512)	(2,779)	(2,490)	(197)	150		$ (296)
Total equity	38,539	38,312	37,460		35,887	36,059	38,539		36,330	36,163	33,187		31,693
Revision of Prior Period, Accounting Standards Update, Adjustment [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Total Shareholders' Balance, beginning									(64)	35
Shareholders' net income									(131)	54	0
Benefit expense									155	(74)	0
Total assets									(17)	(4)
Total liabilities									47	(39)
Accumulated other comprehensive loss									13	(19)	(12)
Total equity									(64)	35	(12)
Retained Earnings [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Total Shareholders' Balance, beginning	32,103	31,608	30,707	29,611	28,847	28,096	$ 32,103	$ 29,611	29,647	27,142	23,802		$ 22,538
Shareholders' net income	$ 1,289	$ 1,853	$ 1,989	$ 1,603	$ 1,637	$ 1,789			$ 5,894	6,158	$ 4,572
Accounting Standards Update 2020-06 [Member] | Cumulative Effect, Period of Adoption, Adjustment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Total equity										(23)
Accounting Standards Update 2020-06 [Member] | Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Long-term Debt										31		$ 31
Deferred Tax and Other Liabilities, Noncurrent										(8)		(8)
Total Shareholders' Balance, beginning										$ (23)		$ (23)
Leasehold Improvements [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Depreciation Methods									Depreciation Method, Straight-Line [Member]
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									3 years
Maximum [Member] | Buildings and Improvements [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life									30 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