Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	$ 1,014	$ 905	$ 805
Gross future policy benefits, losses, claims and expense losses	11,418	10,266	9,615
Gross unearned premium	3,689	3,271	2,960
Other policy claims and benefits payable	41	53	46
Earned premiums	7,219	6,482	5,980
Investment income, net of expenses	680	623	589
Benefits, claims losses and settlement expense	5,012	3,936	4,134
Amortization of deferred policy acquisition costs	1,347	1,197	1,106
Underwriting, acquisition and insurance expenses	815	754	723
Net written premiums	7,309	6,481	5,866
Commercial Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	372	345	317
Gross future policy benefits, losses, claims and expense losses	5,568	5,007	4,881
Gross unearned premium	2,021	1,857	1,714
Earned premiums	4,024	3,674	3,476
Benefits, claims losses and settlement expense	2,761	1,940	2,336
Amortization of deferred policy acquisition costs	769	703	664
Underwriting, acquisition and insurance expenses	460	437	415
Net written premiums	4,159	3,811	3,534
Personal Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	168	145	136
Gross future policy benefits, losses, claims and expense losses	916	814	739
Gross unearned premium	994	850	797
Earned premiums	1,689	1,542	1,463
Benefits, claims losses and settlement expense	1,166	992	977
Amortization of deferred policy acquisition costs	311	276	256
Underwriting, acquisition and insurance expenses	198	181	187
Net written premiums	1,831	1,594	1,503
Excess and Surplus Lines Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	37	33	28
Gross future policy benefits, losses, claims and expense losses	753	577	446
Gross unearned premium	236	207	175
Earned premiums	485	398	325
Benefits, claims losses and settlement expense	315	250	199
Amortization of deferred policy acquisition costs	79	63	51
Underwriting, acquisition and insurance expenses	45	43	43
Net written premiums	502	426	348
Reinsurance assumed and other non segment
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	105	79	61
Gross future policy benefits, losses, claims and expense losses	1,099	831	611
Gross unearned premium	437	356	273
Earned premiums	726	570	427
Benefits, claims losses and settlement expense	474	414	325
Amortization of deferred policy acquisition costs	144	109	86
Underwriting, acquisition and insurance expenses	72	55	42
Net written premiums	815	648	479
Property, Liability and Casualty Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	682	602	542
Gross future policy benefits, losses, claims and expense losses	8,336	7,229	6,677
Gross unearned premium	3,688	3,270	2,959
Earned premiums	6,924	6,184	5,691
Investment income, net of expenses	509	457	431
Benefits, claims losses and settlement expense	4,716	3,596	3,837
Amortization of deferred policy acquisition costs	1,303	1,151	1,057
Underwriting, acquisition and insurance expenses	775	716	687
Net written premiums	7,307	6,479	5,864
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	332	303	263
Gross future policy benefits, losses, claims and expense losses	3,082	3,037	2,938
Gross unearned premium	1	1	1
Other policy claims and benefits payable	41	53	46
Earned premiums	295	298	289
Investment income, net of expenses	171	166	158
Benefits, claims losses and settlement expense	296	340	297
Amortization of deferred policy acquisition costs	44	46	49
Underwriting, acquisition and insurance expenses	$ 40	$ 38	$ 36