Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	$ 637	$ 616	$ 578
Gross future policy benefits, losses, claims and expense losses	7,728	7,265	6,957
Gross unearned premium	2,307	2,201	2,082
Other policy claims and benefits payable	28	36	25
Earned premiums	4,710	4,480	4,243
Investment income, net of expenses	539	518	502
Benefits, claims losses and settlement expense	3,107	2,808	2,856
Amortization of deferred policy acquisition costs	863	829	807
Underwriting, acquisition and insurance expenses	602	558	494
Net written premiums	4,582	4,363	4,146
Commercial Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	271	264	257
Gross future policy benefits, losses, claims and expense losses	4,179	3,925	3,797
Gross unearned premium	1,510	1,472	1,441
Earned premiums	3,089	2,996	2,856
Benefits, claims losses and settlement expense	1,928	1,708	1,812
Amortization of deferred policy acquisition costs	570	552	537
Underwriting, acquisition and insurance expenses	412	395	365
Net written premiums	3,122	3,025	2,922
Personal Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	110	103	108
Gross future policy benefits, losses, claims and expense losses	569	498	439
Gross unearned premium	629	593	562
Earned premiums	1,161	1,097	1,041
Benefits, claims losses and settlement expense	840	789	740
Amortization of deferred policy acquisition costs	209	210	209
Underwriting, acquisition and insurance expenses	128	113	84
Net written premiums	1,198	1,128	1,068
Excess and Surplus Lines Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	16	15	14
Gross future policy benefits, losses, claims and expense losses	241	227	202
Gross unearned premium	93	87	78
Earned premiums	183	168	148
Benefits, claims losses and settlement expense	68	70	75
Amortization of deferred policy acquisition costs	31	28	24
Underwriting, acquisition and insurance expenses	23	20	19
Net written premiums	189	175	153
Reinsurance assumed and other non segment [Member]
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	11	6	0
Gross future policy benefits, losses, claims and expense losses	46	10	0
Gross unearned premium	74	48	0
Earned premiums	49	10	0
Benefits, claims losses and settlement expense	25	5	0
Amortization of deferred policy acquisition costs	10	2	0
Underwriting, acquisition and insurance expenses	6	1	0
Net written premiums	71	33	0
Property, Liability and Casualty Insurance Product Line [Member]
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	408	388	379
Gross future policy benefits, losses, claims and expense losses	5,035	4,660	4,438
Gross unearned premium	2,306	2,200	2,081
Earned premiums	4,482	4,271	4,045
Investment income, net of expenses	384	368	358
Benefits, claims losses and settlement expense	2,861	2,572	2,627
Amortization of deferred policy acquisition costs	820	792	770
Underwriting, acquisition and insurance expenses	569	529	468
Net written premiums	4,580	4,361	4,143
Accident Health Insurance Segment [Member]
Supplementary Insurance Information, by Segment
Net written premiums	2	2	3
Life Insurance Segment
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	229	228	199
Gross future policy benefits, losses, claims and expense losses	2,693	2,605	2,519
Gross unearned premium	1	1	1
Other policy claims and benefits payable	28	36	25
Earned premiums	228	209	198
Investment income, net of expenses	155	150	144
Benefits, claims losses and settlement expense	246	236	229
Amortization of deferred policy acquisition costs	43	37	37
Underwriting, acquisition and insurance expenses	$ 33	$ 29	$ 26