Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	$ 805	$ 774	$ 738
Gross future policy benefits, losses, claims and expense losses	9,615	8,947	8,448
Gross unearned premium	2,960	2,788	2,516
Other policy claims and benefits payable	46	35	38
Earned premiums	5,980	5,604	5,170
Investment income, net of expenses	589	571	554
Benefits, claims losses and settlement expense	4,134	3,638	3,490
Amortization of deferred policy acquisition costs	1,106	1,034	946
Underwriting, acquisition and insurance expenses	723	704	651
Net written premiums	5,866	5,518	5,033
Commercial Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	317	311	291
Gross future policy benefits, losses, claims and expense losses	4,881	4,569	4,466
Gross unearned premium	1,714	1,665	1,576
Earned premiums	3,476	3,319	3,218
Benefits, claims losses and settlement expense	2,336	2,030	2,049
Amortization of deferred policy acquisition costs	664	631	608
Underwriting, acquisition and insurance expenses	415	422	415
Net written premiums	3,534	3,410	3,245
Personal Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	136	130	126
Gross future policy benefits, losses, claims and expense losses	739	687	679
Gross unearned premium	797	757	725
Earned premiums	1,463	1,404	1,336
Benefits, claims losses and settlement expense	977	985	972
Amortization of deferred policy acquisition costs	256	251	242
Underwriting, acquisition and insurance expenses	187	164	147
Net written premiums	1,503	1,435	1,378
Excess and Surplus Lines Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	28	25	21
Gross future policy benefits, losses, claims and expense losses	446	351	298
Gross unearned premium	175	152	123
Earned premiums	325	278	234
Benefits, claims losses and settlement expense	199	142	104
Amortization of deferred policy acquisition costs	51	47	39
Underwriting, acquisition and insurance expenses	43	38	29
Net written premiums	348	303	249
Reinsurance assumed and other non segment
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	61	46	26
Gross future policy benefits, losses, claims and expense losses	611	481	203
Gross unearned premium	273	213	91
Earned premiums	427	333	132
Benefits, claims losses and settlement expense	325	195	98
Amortization of deferred policy acquisition costs	86	57	18
Underwriting, acquisition and insurance expenses	42	42	24
Net written premiums	479	368	158
Property, Liability and Casualty Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	542	512	464
Gross future policy benefits, losses, claims and expense losses	6,677	6,088	5,646
Gross unearned premium	2,959	2,787	2,515
Earned premiums	5,691	5,334	4,920
Investment income, net of expenses	431	419	401
Benefits, claims losses and settlement expense	3,837	3,352	3,223
Amortization of deferred policy acquisition costs	1,057	986	907
Underwriting, acquisition and insurance expenses	687	666	615
Net written premiums	5,864	5,516	5,030
Accident and Health Insurance
Supplementary Insurance Information, by Segment
Net written premiums	2	2	3
Life Insurance Segment
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	263	262	274
Gross future policy benefits, losses, claims and expense losses	2,938	2,859	2,802
Gross unearned premium	1	1	1
Other policy claims and benefits payable	46	35	38
Earned premiums	289	270	250
Investment income, net of expenses	158	152	153
Benefits, claims losses and settlement expense	297	286	267
Amortization of deferred policy acquisition costs	49	48	39
Underwriting, acquisition and insurance expenses	$ 36	$ 38	$ 36