Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	$ 905	$ 805	$ 774
Gross future policy benefits, losses, claims and expense losses	10,266	9,615	8,947
Gross unearned premium	3,271	2,960	2,788
Other policy claims and benefits payable	53	46	35
Earned premiums	6,482	5,980	5,604
Investment income, net of expenses	623	589	571
Benefits, claims losses and settlement expense	3,936	4,134	3,638
Amortization of deferred policy acquisition costs	1,197	1,106	1,034
Underwriting, acquisition and insurance expenses	754	723	704
Net written premiums	6,481	5,866	5,518
Commercial Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	345	317	311
Gross future policy benefits, losses, claims and expense losses	5,007	4,881	4,569
Gross unearned premium	1,857	1,714	1,665
Earned premiums	3,674	3,476	3,319
Benefits, claims losses and settlement expense	1,940	2,336	2,030
Amortization of deferred policy acquisition costs	703	664	631
Underwriting, acquisition and insurance expenses	437	415	422
Net written premiums	3,811	3,534	3,410
Personal Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	145	136	130
Gross future policy benefits, losses, claims and expense losses	814	739	687
Gross unearned premium	850	797	757
Earned premiums	1,542	1,463	1,404
Benefits, claims losses and settlement expense	992	977	985
Amortization of deferred policy acquisition costs	276	256	251
Underwriting, acquisition and insurance expenses	181	187	164
Net written premiums	1,594	1,503	1,435
Excess and Surplus Lines Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	33	28	25
Gross future policy benefits, losses, claims and expense losses	577	446	351
Gross unearned premium	207	175	152
Earned premiums	398	325	278
Benefits, claims losses and settlement expense	250	199	142
Amortization of deferred policy acquisition costs	63	51	47
Underwriting, acquisition and insurance expenses	43	43	38
Net written premiums	426	348	303
Reinsurance assumed and other non segment
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	79	61	46
Gross future policy benefits, losses, claims and expense losses	831	611	481
Gross unearned premium	356	273	213
Earned premiums	570	427	333
Benefits, claims losses and settlement expense	414	325	195
Amortization of deferred policy acquisition costs	109	86	57
Underwriting, acquisition and insurance expenses	55	42	42
Net written premiums	648	479	368
Property, Liability and Casualty Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	602	542	512
Gross future policy benefits, losses, claims and expense losses	7,229	6,677	6,088
Gross unearned premium	3,270	2,959	2,787
Earned premiums	6,184	5,691	5,334
Investment income, net of expenses	457	431	419
Benefits, claims losses and settlement expense	3,596	3,837	3,352
Amortization of deferred policy acquisition costs	1,151	1,057	986
Underwriting, acquisition and insurance expenses	716	687	666
Net written premiums	6,479	5,864	5,516
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	303	263	262
Gross future policy benefits, losses, claims and expense losses	3,037	2,938	2,859
Gross unearned premium	1	1	1
Other policy claims and benefits payable	53	46	35
Earned premiums	298	289	270
Investment income, net of expenses	166	158	152
Benefits, claims losses and settlement expense	340	297	286
Amortization of deferred policy acquisition costs	46	49	48
Underwriting, acquisition and insurance expenses	$ 38	$ 36	$ 38