Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	$ 670	$ 637	$ 616
Gross future policy benefits, losses, claims and expense losses	7,972	7,728	7,265
Gross unearned premium	2,404	2,307	2,201
Other policy claims and benefits payable	30	28	36
Earned premiums	4,954	4,710	4,480
Investment income, net of expenses	547	539	518
Benefits, claims losses and settlement expense	3,390	3,107	2,808
Amortization of deferred policy acquisition costs	913	863	829
Underwriting, acquisition and insurance expenses	633	602	558
Net written premiums	4,843	4,582	4,363
Commercial Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	284	271	264
Gross future policy benefits, losses, claims and expense losses	4,236	4,179	3,925
Gross unearned premium	1,548	1,510	1,472
Earned premiums	3,165	3,089	2,996
Benefits, claims losses and settlement expense	2,042	1,928	1,708
Amortization of deferred policy acquisition costs	590	570	552
Underwriting, acquisition and insurance expenses	419	412	395
Net written premiums	3,202	3,122	3,025
Personal Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	121	110	103
Gross future policy benefits, losses, claims and expense losses	587	569	498
Gross unearned premium	683	629	593
Earned premiums	1,241	1,161	1,097
Benefits, claims losses and settlement expense	918	840	789
Amortization of deferred policy acquisition costs	225	209	210
Underwriting, acquisition and insurance expenses	135	128	113
Net written premiums	1,294	1,198	1,128
Excess and Surplus Lines Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	17	16	15
Gross future policy benefits, losses, claims and expense losses	264	241	227
Gross unearned premium	105	93	87
Earned premiums	209	183	168
Benefits, claims losses and settlement expense	86	68	70
Amortization of deferred policy acquisition costs	35	31	28
Underwriting, acquisition and insurance expenses	28	23	20
Net written premiums	219	189	175
Reinsurance assumed and other non segment [Member]
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	16	11	6
Gross future policy benefits, losses, claims and expense losses	132	46	10
Gross unearned premium	67	74	48
Earned premiums	107	49	10
Benefits, claims losses and settlement expense	92	25	5
Amortization of deferred policy acquisition costs	17	10	2
Underwriting, acquisition and insurance expenses	18	6	1
Net written premiums	125	71	33
Property, Liability and Casualty Insurance Product Line [Member]
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	438	408	388
Gross future policy benefits, losses, claims and expense losses	5,219	5,035	4,660
Gross unearned premium	2,403	2,306	2,200
Earned premiums	4,722	4,482	4,271
Investment income, net of expenses	392	384	368
Benefits, claims losses and settlement expense	3,138	2,861	2,572
Amortization of deferred policy acquisition costs	867	820	792
Underwriting, acquisition and insurance expenses	600	569	529
Net written premiums	4,840	4,580	4,361
Accident Health Insurance Segment [Member]
Supplementary Insurance Information, by Segment
Net written premiums	3	2	2
Life Insurance Segment
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	232	229	228
Gross future policy benefits, losses, claims and expense losses	2,753	2,693	2,605
Gross unearned premium	1	1	1
Other policy claims and benefits payable	30	28	36
Earned premiums	232	228	209
Investment income, net of expenses	155	155	150
Benefits, claims losses and settlement expense	252	246	236
Amortization of deferred policy acquisition costs	46	43	37
Underwriting, acquisition and insurance expenses	$ 33	$ 33	$ 29