Supplementary Insurance Information (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	$ 578	$ 565	$ 470
Gross future policy benefits, losses, claims and expense losses	6,957	6,682	6,510
Gross unearned premium	2,082	1,976	1,792
Other policy claims and benefits payable	25	19	15
Earned premiums	4,243	3,902	3,522
Investment income, net of expenses	502	488	488
Benefits, claims losses and settlement expense	2,856	2,505	2,322
Amortization of deferred policy acquisition costs	807	758	707
Underwriting, acquisition and insurance expenses	494	485	448
Net written premiums	4,146	3,895	3,485
Commercial Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	257	251	235
Gross future policy benefits, losses, claims and expense losses	3,797	3,667	3,645
Gross unearned premium	1,441	1,372	1,246
Earned premiums	2,856	2,636	2,383
Benefits, claims losses and settlement expense	1,812	1,596	1,420
Amortization of deferred policy acquisition costs	537	514	462
Underwriting, acquisition and insurance expenses	365	343	324
Net written premiums	2,922	2,760	2,459
Personal Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	108	104	93
Gross future policy benefits, losses, claims and expense losses	439	417	398
Gross unearned premium	562	535	490
Earned premiums	1,041	961	868
Benefits, claims losses and settlement expense	740	639	652
Amortization of deferred policy acquisition costs	209	192	183
Underwriting, acquisition and insurance expenses	84	98	78
Net written premiums	1,068	1,005	918
Excess And Surplus Lines Insurance
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	14	11	9
Gross future policy benefits, losses, claims and expense losses	202	157	126
Gross unearned premium	78	67	54
Earned premiums	148	116	93
Benefits, claims losses and settlement expense	75	66	65
Amortization of deferred policy acquisition costs	24	21	17
Underwriting, acquisition and insurance expenses	19	15	12
Net written premiums	153	128	105
Property, Liability and Casualty Insurance Product Line [Member]
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	379	366	337
Gross future policy benefits, losses, claims and expense losses	4,438	4,241	4,169
Gross unearned premium	2,081	1,974	1,790
Earned premiums	4,045	3,713	3,344
Investment income, net of expenses	358	348	351
Benefits, claims losses and settlement expense	2,627	2,301	2,137
Amortization of deferred policy acquisition costs	770	727	662
Underwriting, acquisition and insurance expenses	468	456	414
Net written premiums	4,143	3,893	3,482
Accident Health Insurance Segment [Member]
Supplementary Insurance Information, by Segment
Net written premiums	3	2	3
Life Insurance Segment
Supplementary Insurance Information, by Segment
Deferred policy acquisition costs	199	199	133
Gross future policy benefits, losses, claims and expense losses	2,519	2,441	2,341
Gross unearned premium	1	2	2
Other policy claims and benefits payable	25	19	15
Earned premiums	198	189	178
Investment income, net of expenses	144	140	137
Benefits, claims losses and settlement expense	229	204	185
Amortization of deferred policy acquisition costs	37	31	45
Underwriting, acquisition and insurance expenses	$ 26	$ 29	$ 34