Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	$ 1,093	$ 1,013	$ 916
Gross future policy benefits, losses, claims and expense losses	12,069	11,374	10,727
Gross unearned premium	4,119	3,689	3,271
Other policy claims and benefits payable	49	41	53
Earned premiums	7,958	7,225	6,478
Investment income, net of expenses	786	680	623
Benefits, claims losses and settlement expense	5,274	5,019	3,909
Amortization of deferred policy acquisition costs	1,450	1,330	1,176
Underwriting, acquisition and insurance expenses	934	832	770
Net written premiums	8,048	7,309	6,481
Commercial Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	387	372	345
Gross future policy benefits, losses, claims and expense losses	5,887	5,568	5,007
Gross unearned premium	2,111	2,021	1,857
Earned premiums	4,264	4,024	3,674
Benefits, claims losses and settlement expense	2,787	2,761	1,940
Amortization of deferred policy acquisition costs	808	769	703
Underwriting, acquisition and insurance expenses	505	460	437
Net written premiums	4,336	4,159	3,811
Personal Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	210	168	145
Gross future policy benefits, losses, claims and expense losses	990	916	814
Gross unearned premium	1,253	994	850
Earned premiums	2,044	1,689	1,542
Benefits, claims losses and settlement expense	1,442	1,166	992
Amortization of deferred policy acquisition costs	367	311	276
Underwriting, acquisition and insurance expenses	243	198	181
Net written premiums	2,302	1,831	1,594
Excess and Surplus Lines Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	43	37	33
Gross future policy benefits, losses, claims and expense losses	932	753	577
Gross unearned premium	273	236	207
Earned premiums	542	485	398
Benefits, claims losses and settlement expense	350	315	250
Amortization of deferred policy acquisition costs	89	79	63
Underwriting, acquisition and insurance expenses	52	45	43
Net written premiums	570	502	426
Reinsurance assumed and other non segment
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	109	105	79
Gross future policy benefits, losses, claims and expense losses	1,166	1,099	831
Gross unearned premium	481	437	356
Earned premiums	795	726	570
Benefits, claims losses and settlement expense	379	474	414
Amortization of deferred policy acquisition costs	157	144	109
Underwriting, acquisition and insurance expenses	76	72	55
Net written premiums	838	815	648
Property, Liability and Casualty Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	749	682	602
Gross future policy benefits, losses, claims and expense losses	8,975	8,336	7,229
Gross unearned premium	4,118	3,688	3,270
Earned premiums	7,645	6,924	6,184
Investment income, net of expenses	602	509	457
Benefits, claims losses and settlement expense	4,958	4,716	3,596
Amortization of deferred policy acquisition costs	1,421	1,303	1,151
Underwriting, acquisition and insurance expenses	876	775	716
Net written premiums	8,046	7,307	6,479
Accident and Health Insurance
Supplementary Insurance Information, by Segment
Net written premiums	2	2	2
Life Insurance Segment
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	344	331	314
Gross future policy benefits, losses, claims and expense losses	3,094	3,038	3,498
Gross unearned premium	1	1	1
Other policy claims and benefits payable	49	41	53
Earned premiums	313	301	294
Investment income, net of expenses	184	171	166
Benefits, claims losses and settlement expense	316	303	313
Amortization of deferred policy acquisition costs	29	27	25
Underwriting, acquisition and insurance expenses	$ 58	$ 57	$ 54