Supplementary Insurance Information (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	$ 470		$ 477		$ 458
Gross future policy benefits, losses, claims and expense losses	6,510	[1]	6,537	[1]	6,210	[1]
Gross unearned premium	1,792	[1]	1,633	[1]	1,553	[1]
Other policy claims and benefits payable	15	[1]	16	[1]	24	[1]
Earned premiums	3,522		3,194		3,082
Investment income, net of expenses	488		484		477
Benefits, claims losses and settlement expense	2,322		2,524		2,180
Amortization of deferred policy acquisition costs	707	[2]	631	[2]	608	[2]
Underwriting, acquisition and insurance expenses	448	[2]	408	[2]	416	[2]
Net written premiums	3,485		3,101		2,966
Commercial Lines [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	235		204		204
Gross future policy benefits, losses, claims and expense losses	3,645		3,780		3,728
Gross unearned premium	1,246		1,149		1,116
Earned premiums	2,383		2,197		2,154
Benefits, claims losses and settlement expense	1,420		1,570		1,437
Amortization of deferred policy acquisition costs	462		441		424
Underwriting, acquisition and insurance expenses	324		291		281
Net written premiums	2,459		2,218		2,155
Personal Lines [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	93		92		76
Gross future policy benefits, losses, claims and expense losses	398		419		353
Gross unearned premium	490		440		401
Earned premiums	868		762		721
Benefits, claims losses and settlement expense	652		723		537
Amortization of deferred policy acquisition costs	183		144		139
Underwriting, acquisition and insurance expenses	78		78		102
Net written premiums	918		801		750
Excess And Surplus Lines Insurance [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	9		7		6
Gross future policy benefits, losses, claims and expense losses	126		81		56
Gross unearned premium	54		42		34
Earned premiums	93		70		49
Benefits, claims losses and settlement expense	65		42		41
Amortization of deferred policy acquisition costs	17		13		9
Underwriting, acquisition and insurance expenses	12		9		7
Net written premiums	105		79		58
Property, Liability And Casualty Insurance Segment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	337		303		286
Gross future policy benefits, losses, claims and expense losses	4,169		4,280		4,137
Gross unearned premium	1,790		1,631		1,551
Earned premiums	3,344		3,029		2,924
Investment income, net of expenses	351	[3]	350	[3]	348	[3]
Benefits, claims losses and settlement expense	2,137		2,335		2,015
Amortization of deferred policy acquisition costs	662		598		572
Underwriting, acquisition and insurance expenses	414		378		390
Net written premiums	3,482		3,098		2,963
Accident Health Insurance Segment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Net written premiums	3		3		3
Life Insurance Segment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	133		174		172
Gross future policy benefits, losses, claims and expense losses	2,341		2,257		2,073
Gross unearned premium	2		2		2
Other policy claims and benefits payable	15		16		24
Earned premiums	178		165		158
Investment income, net of expenses	137		134		129
Benefits, claims losses and settlement expense	185		189		170
Amortization of deferred policy acquisition costs	45		33		36
Underwriting, acquisition and insurance expenses	34		30		26
Consolidation, Eliminations [Member]
Supplementary Insurance Information, by Segment [Line Items]
Benefits, claims losses and settlement expense					$ (5)

[1] The sum of gross future policy benefits, losses, claims and expense losses, gross unearned premium and other policy claims and benefits payable is equal to the sum of Loss and loss expense reserves, Life policy reserves and investment contract reserves and Unearned premiums reported in the company’s consolidated balance sheets, Page 108.

[2] The sum of amortization of deferred policy acquisition costs and other underwriting and insurance expenses is equal to Underwriting, acquisition and insurance expenses in the consolidated statements of income.

[3] This segment information is not regularly allocated to segments and not reviewed by company management in making decisions about resources to be allocated to the segments or to assess their performance.