Basis of Presentation and Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Customer funds and cash and cash equivalents on deposit for regulatory requirements	$ 348	$ 409
Securities lending transactions ratio of fair value of collateral held to fair value of securities loaned	102.00%	102.00%
Premium receivable, allowance for doubtful accounts	$ 167	$ 183
Self-funded receivables, allowance for doubtful accounts	145	115
Allowance for doubtful accounts, other receivables	1,509	1,385
Settlement	264
Release of allowance for doubtful accounts	$ 129
Employee stock purchase plan, purchase price per share as a percent of closing price	90.00%
Advertising and marketing expense	$ 395	540	$ 599
Operating lease, impairment loss	7	17	23
Total shareholders	43,882	41,315
Shareholders' net income	5,662	5,980	5,987
Benefit expense	148,223	127,567	$ 124,330
Total assets	121,494	116,889
Total liabilities	77,468	75,463
Accumulated other comprehensive loss	(451)	$ (1,147)
Services Rendered in Prior Year
Property, Plant and Equipment [Line Items]
Settlement	154
Services Rendered in Current Year
Property, Plant and Equipment [Line Items]
Settlement	$ 110
Revision of Prior Period, Accounting Standards Update, Adjustment
Property, Plant and Equipment [Line Items]
Total shareholders				$ (64)
Minimum | Building and improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	15 years
Minimum | Computer equipment and software
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	3 years
Minimum | Internal-use software
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	3 years
Maximum | Building and improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	30 years
Maximum | Computer equipment and software
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5 years
Maximum | Furniture and other equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	7 years
Maximum | Internal-use software
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	10 years