Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	$ 616	$ 578	$ 565
Gross future policy benefits, losses, claims and expense losses	7,265	6,957	6,682
Gross unearned premium	2,201	2,082	1,976
Other policy claims and benefits payable	36	25	19
Earned premiums	4,480	4,243	3,902
Investment income, net of expenses	518	502	488
Benefits, claims losses and settlement expense	2,808	2,856	2,505
Amortization of deferred policy acquisition costs	829	807	758
Underwriting, acquisition and insurance expenses	558	494	485
Net written premiums	4,363	4,146	3,895
Commercial Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	264	257	251
Gross future policy benefits, losses, claims and expense losses	3,925	3,797	3,667
Gross unearned premium	1,472	1,441	1,372
Earned premiums	2,996	2,856	2,636
Benefits, claims losses and settlement expense	1,708	1,812	1,596
Amortization of deferred policy acquisition costs	552	537	514
Underwriting, acquisition and insurance expenses	395	365	343
Net written premiums	3,025	2,922	2,760
Personal Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	103	108	104
Gross future policy benefits, losses, claims and expense losses	498	439	417
Gross unearned premium	593	562	535
Earned premiums	1,097	1,041	961
Benefits, claims losses and settlement expense	789	740	639
Amortization of deferred policy acquisition costs	210	209	192
Underwriting, acquisition and insurance expenses	113	84	98
Net written premiums	1,128	1,068	1,005
Excess And Surplus Lines Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	15	14	11
Gross future policy benefits, losses, claims and expense losses	227	202	157
Gross unearned premium	87	78	67
Earned premiums	168	148	116
Benefits, claims losses and settlement expense	70	75	66
Amortization of deferred policy acquisition costs	28	24	21
Underwriting, acquisition and insurance expenses	20	19	15
Net written premiums	175	153	128
Reinsurance assumed and other non segment [Member]
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	6	0	0
Gross future policy benefits, losses, claims and expense losses	10	0	0
Gross unearned premium	48	0	0
Earned premiums	10	0	0
Benefits, claims losses and settlement expense	5	0	0
Amortization of deferred policy acquisition costs	2	0	0
Underwriting, acquisition and insurance expenses	1	0	0
Net written premiums	33	0	0
Property, Liability and Casualty Insurance Product Line [Member]
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	388	379	366
Gross future policy benefits, losses, claims and expense losses	4,660	4,438	4,241
Gross unearned premium	2,200	2,081	1,974
Earned premiums	4,271	4,045	3,713
Investment income, net of expenses	368	358	348
Benefits, claims losses and settlement expense	2,572	2,627	2,301
Amortization of deferred policy acquisition costs	792	770	727
Underwriting, acquisition and insurance expenses	529	468	456
Net written premiums	4,361	4,143	3,893
Accident Health Insurance Segment [Member]
Supplementary Insurance Information, by Segment
Net written premiums	2	3	2
Life Insurance Segment
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	228	199	199
Gross future policy benefits, losses, claims and expense losses	2,605	2,519	2,441
Gross unearned premium	1	1	2
Other policy claims and benefits payable	36	25	19
Earned premiums	209	198	189
Investment income, net of expenses	150	144	140
Benefits, claims losses and settlement expense	236	229	204
Amortization of deferred policy acquisition costs	37	37	31
Underwriting, acquisition and insurance expenses	$ 29	$ 26	$ 29