Supplementary Insurance Information (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	$ 565	$ 470	$ 477
Gross future policy benefits, losses, claims and expense losses	6,682	6,510	6,537
Gross unearned premium	1,976	1,792	1,633
Other policy claims and benefits payable	19	15	16
Earned premiums	3,902	3,522	3,194
Investment income, net of expenses	488	488	484
Benefits, claims losses and settlement expense	2,505	2,322	2,524
Amortization of deferred policy acquisition costs	758	707	631
Underwriting, acquisition and insurance expenses	485	448	408
Net written premiums	3,895	3,485	3,101
Commercial Lines [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	251	235	204
Gross future policy benefits, losses, claims and expense losses	3,667	3,645	3,780
Gross unearned premium	1,372	1,246	1,149
Earned premiums	2,636	2,383	2,197
Benefits, claims losses and settlement expense	1,596	1,420	1,570
Amortization of deferred policy acquisition costs	514	462	441
Underwriting, acquisition and insurance expenses	343	324	291
Net written premiums	2,760	2,459	2,218
Personal Lines [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	104	93	92
Gross future policy benefits, losses, claims and expense losses	417	398	419
Gross unearned premium	535	490	440
Earned premiums	961	868	762
Benefits, claims losses and settlement expense	639	652	723
Amortization of deferred policy acquisition costs	192	183	144
Underwriting, acquisition and insurance expenses	98	78	78
Net written premiums	1,005	918	801
Excess And Surplus Lines Insurance [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	11	9	7
Gross future policy benefits, losses, claims and expense losses	157	126	81
Gross unearned premium	67	54	42
Earned premiums	116	93	70
Benefits, claims losses and settlement expense	66	65	42
Amortization of deferred policy acquisition costs	21	17	13
Underwriting, acquisition and insurance expenses	15	12	9
Net written premiums	128	105	79
Property, Liability and Casualty Insurance Product Line [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	366	337	303
Gross future policy benefits, losses, claims and expense losses	4,241	4,169	4,280
Gross unearned premium	1,974	1,790	1,631
Earned premiums	3,713	3,344	3,029
Investment income, net of expenses	348	351	350
Benefits, claims losses and settlement expense	2,301	2,137	2,335
Amortization of deferred policy acquisition costs	727	662	598
Underwriting, acquisition and insurance expenses	456	414	378
Net written premiums	3,893	3,482	3,098
Accident Health Insurance Segment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Net written premiums	2	3	3
Life Insurance Segment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	199	133	174
Gross future policy benefits, losses, claims and expense losses	2,441	2,341	2,257
Gross unearned premium	2	2	2
Other policy claims and benefits payable	19	15	16
Earned premiums	189	178	165
Investment income, net of expenses	140	137	134
Benefits, claims losses and settlement expense	204	185	189
Amortization of deferred policy acquisition costs	31	45	33
Underwriting, acquisition and insurance expenses	$ 29	$ 34	$ 30