Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	$ 738	$ 670	$ 637
Gross future policy benefits, losses, claims and expense losses	8,448	7,972	7,728
Gross unearned premium	2,516	2,404	2,307
Other policy claims and benefits payable	38	30	28
Earned premiums	5,170	4,954	4,710
Investment income, net of expenses	554	547	539
Benefits, claims losses and settlement expense	3,490	3,390	3,107
Amortization of deferred policy acquisition costs	946	913	863
Underwriting, acquisition and insurance expenses	651	633	602
Net written premiums	5,033	4,843	4,582
Commercial Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	291	284	271
Gross future policy benefits, losses, claims and expense losses	4,466	4,236	4,179
Gross unearned premium	1,576	1,548	1,510
Earned premiums	3,218	3,165	3,089
Benefits, claims losses and settlement expense	2,049	2,042	1,928
Amortization of deferred policy acquisition costs	608	590	570
Underwriting, acquisition and insurance expenses	415	419	412
Net written premiums	3,245	3,202	3,122
Personal Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	126	121	110
Gross future policy benefits, losses, claims and expense losses	679	587	569
Gross unearned premium	725	683	629
Earned premiums	1,336	1,241	1,161
Benefits, claims losses and settlement expense	972	918	840
Amortization of deferred policy acquisition costs	242	225	209
Underwriting, acquisition and insurance expenses	147	135	128
Net written premiums	1,378	1,294	1,198
Excess and Surplus Lines Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	21	17	16
Gross future policy benefits, losses, claims and expense losses	298	264	241
Gross unearned premium	123	105	93
Earned premiums	234	209	183
Benefits, claims losses and settlement expense	104	86	68
Amortization of deferred policy acquisition costs	39	35	31
Underwriting, acquisition and insurance expenses	29	28	23
Net written premiums	249	219	189
Reinsurance assumed and other non segment [Member]
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	26	16	11
Gross future policy benefits, losses, claims and expense losses	203	132	46
Gross unearned premium	91	67	74
Earned premiums	132	107	49
Benefits, claims losses and settlement expense	98	92	25
Amortization of deferred policy acquisition costs	18	17	10
Underwriting, acquisition and insurance expenses	24	18	6
Net written premiums	158	125	71
Property, Liability and Casualty Insurance Product Line [Member]
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	464	438	408
Gross future policy benefits, losses, claims and expense losses	5,646	5,219	5,035
Gross unearned premium	2,515	2,403	2,306
Earned premiums	4,920	4,722	4,482
Investment income, net of expenses	401	392	384
Benefits, claims losses and settlement expense	3,223	3,138	2,861
Amortization of deferred policy acquisition costs	907	867	820
Underwriting, acquisition and insurance expenses	615	600	569
Net written premiums	5,030	4,840	4,580
Accident Health Insurance Segment [Member]
Supplementary Insurance Information, by Segment
Net written premiums	3	3	2
Life Insurance Segment
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	274	232	229
Gross future policy benefits, losses, claims and expense losses	2,802	2,753	2,693
Gross unearned premium	1	1	1
Other policy claims and benefits payable	38	30	28
Earned premiums	250	232	228
Investment income, net of expenses	153	155	155
Benefits, claims losses and settlement expense	267	252	246
Amortization of deferred policy acquisition costs	39	46	43
Underwriting, acquisition and insurance expenses	$ 36	$ 33	$ 33