Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	$ 774	$ 738	$ 670
Gross future policy benefits, losses, claims and expense losses	8,947	8,448	7,972
Gross unearned premium	2,788	2,516	2,404
Other policy claims and benefits payable	35	38	30
Earned premiums	5,604	5,170	4,954
Investment income, net of expenses	571	554	547
Benefits, claims losses and settlement expense	3,638	3,490	3,390
Amortization of deferred policy acquisition costs	1,034	946	913
Underwriting, acquisition and insurance expenses	704	651	633
Net written premiums	5,518	5,033	4,843
Commercial Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	311	291	284
Gross future policy benefits, losses, claims and expense losses	4,569	4,466	4,236
Gross unearned premium	1,665	1,576	1,548
Earned premiums	3,319	3,218	3,165
Benefits, claims losses and settlement expense	2,030	2,049	2,042
Amortization of deferred policy acquisition costs	631	608	590
Underwriting, acquisition and insurance expenses	422	415	419
Net written premiums	3,410	3,245	3,202
Personal Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	130	126	121
Gross future policy benefits, losses, claims and expense losses	687	679	587
Gross unearned premium	757	725	683
Earned premiums	1,404	1,336	1,241
Benefits, claims losses and settlement expense	985	972	918
Amortization of deferred policy acquisition costs	251	242	225
Underwriting, acquisition and insurance expenses	164	147	135
Net written premiums	1,435	1,378	1,294
Excess and Surplus Lines Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	25	21	17
Gross future policy benefits, losses, claims and expense losses	351	298	264
Gross unearned premium	152	123	105
Earned premiums	278	234	209
Benefits, claims losses and settlement expense	142	104	86
Amortization of deferred policy acquisition costs	47	39	35
Underwriting, acquisition and insurance expenses	38	29	28
Net written premiums	303	249	219
Reinsurance assumed and other non segment
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	46	26	16
Gross future policy benefits, losses, claims and expense losses	481	203	132
Gross unearned premium	213	91	67
Earned premiums	333	132	107
Benefits, claims losses and settlement expense	195	98	92
Amortization of deferred policy acquisition costs	57	18	17
Underwriting, acquisition and insurance expenses	42	24	18
Net written premiums	368	158	125
Property, Liability and Casualty Insurance Product Line
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	512	464	438
Gross future policy benefits, losses, claims and expense losses	6,088	5,646	5,219
Gross unearned premium	2,787	2,515	2,403
Earned premiums	5,334	4,920	4,722
Investment income, net of expenses	419	401	392
Benefits, claims losses and settlement expense	3,352	3,223	3,138
Amortization of deferred policy acquisition costs	986	907	867
Underwriting, acquisition and insurance expenses	666	615	600
Net written premiums	5,516	5,030	4,840
Accident Health Insurance Segment
Supplementary Insurance Information, by Segment
Net written premiums	2	3	3
Life Insurance Segment
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	262	274	232
Gross future policy benefits, losses, claims and expense losses	2,859	2,802	2,753
Gross unearned premium	1	1	1
Other policy claims and benefits payable	35	38	30
Earned premiums	270	250	232
Investment income, net of expenses	152	153	155
Benefits, claims losses and settlement expense	286	267	252
Amortization of deferred policy acquisition costs	48	39	46
Underwriting, acquisition and insurance expenses	$ 38	$ 36	$ 33