Segment Information - Reconciliation of Revenue from Segments to Consolidated (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Segment Reporting [Line Items]
Earned premiums and fee income	$ 6,438	$ 6,177	$ 19,035	$ 18,145
Net investment income	850	755	2,384	2,065
Realized Investment Gains (Losses)	(116)	(17)	(104)	(85)
Other Income	48	31	109	83
Total revenues	7,220	6,946	21,424	20,208
P&C Personal Insurance \| AARP Members [Member]
Segment Reporting [Line Items]
Earned premiums and fee income	794	855	2,400	2,520
Operating Segments [Member]
Segment Reporting [Line Items]
Earned premiums and fee income	6,427	6,167	19,003	18,114
Net investment income	832	741	2,332	2,023
Realized Investment Gains (Losses)	(121)	(38)	(130)	(110)
Other Income	24	25	72	71
Total revenues	7,162	6,895	21,277	20,098
Operating Segments [Member] \| P&C Business Insurance
Segment Reporting [Line Items]
Earned premiums and fee income	3,787	3,551	11,046	10,321
Net investment income	589	519	1,650	1,405
Realized Investment Gains (Losses)	(117)	(26)	(124)	(70)
Other Income	0	0	1	2
Total revenues	4,259	4,044	12,573	11,658
Operating Segments [Member] \| P&C Personal Insurance
Segment Reporting [Line Items]
Earned premiums and fee income	921	958	2,748	2,804
Net investment income	67	67	196	182
Realized Investment Gains (Losses)	(15)	(4)	(15)	(10)
Other Income	24	25	71	69
Total revenues	997	1,046	3,000	3,045
Operating Segments [Member] \| Property & Casualty Other Operations [Member]
Segment Reporting [Line Items]
Earned premiums and fee income	0	0	0	0
Net investment income	22	19	64	56
Realized Investment Gains (Losses)	(4)	0	(4)	(2)
Other Income	0	0	0	0
Total revenues	18	19	60	54
Operating Segments [Member] \| Employee Benefits [Member]
Segment Reporting [Line Items]
Earned premiums and fee income	1,719	1,658	5,209	4,989
Net investment income	154	136	422	380
Realized Investment Gains (Losses)	15	(8)	13	(28)
Other Income	0	0	0	0
Total revenues	1,888	1,786	5,644	5,341
Operating Segments [Member] \| Workers' Compensation \| P&C Business Insurance
Segment Reporting [Line Items]
Earned premiums and fee income	954	938	2,808	2,773
Operating Segments [Member] \| General liability \| P&C Business Insurance
Segment Reporting [Line Items]
Earned premiums and fee income	703	633	2,013	1,826
Operating Segments [Member] \| Marine \| P&C Business Insurance
Segment Reporting [Line Items]
Earned premiums and fee income	75	72	220	209
Operating Segments [Member] \| Package business \| P&C Business Insurance
Segment Reporting [Line Items]
Earned premiums and fee income	746	683	2,170	1,961
Operating Segments [Member] \| Property \| P&C Business Insurance
Segment Reporting [Line Items]
Earned premiums and fee income	385	372	1,140	1,066
Operating Segments [Member] \| Property \| P&C Personal Insurance
Segment Reporting [Line Items]
Earned premiums and fee income	334	319	971	907
Operating Segments [Member] \| Professional liability \| P&C Business Insurance
Segment Reporting [Line Items]
Earned premiums and fee income	218	224	643	638
Operating Segments [Member] \| Bond \| P&C Business Insurance
Segment Reporting [Line Items]
Earned premiums and fee income	89	88	262	256
Operating Segments [Member] \| Assumed reinsurance \| P&C Business Insurance
Segment Reporting [Line Items]
Earned premiums and fee income	242	218	708	660
Operating Segments [Member] \| Automobile \| P&C Business Insurance
Segment Reporting [Line Items]
Earned premiums and fee income	375	323	1,082	932
Operating Segments [Member] \| Automobile \| P&C Personal Insurance
Segment Reporting [Line Items]
Earned premiums and fee income	587	639	1,777	1,897
Operating Segments [Member] \| Group disability \| Employee Benefits [Member]
Segment Reporting [Line Items]
Earned premiums and fee income	944	890	2,851	2,689
Operating Segments [Member] \| Group life \| Employee Benefits [Member]
Segment Reporting [Line Items]
Earned premiums and fee income	641	648	1,954	1,942
Operating Segments [Member] \| Other \| Employee Benefits [Member]
Segment Reporting [Line Items]
Earned premiums and fee income	$ 134	$ 120	$ 404	$ 358