Segment Information (Schedule Of Segment Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Segment Reporting Information [Line Items]
Revenues	$ 1,070	[1]	$ 1,035	[1]	$ 1,020	[1]	$ 986	[1]	$ 955	[1]	$ 944	[1]	$ 975	[1]	$ 929	[1]	$ 4,111	[1]	$ 3,803	[1]	$ 3,772
Income (loss) before income taxes	273		152		29		112		186		11		(99)		75		566		173		498
Identifiable assets	16,548								15,635								16,548		15,635
Property Casualty Insurance Segment [Member]
Segment Reporting Information [Line Items]
Identifiable assets	2,395								2,272								2,395		2,272
Commercial Lines [Member]
Segment Reporting Information [Line Items]
Revenues																	2,387		2,200		2,156
Income (loss) before income taxes																	181		(102)		14
Commercial Lines [Member] | Commercial Casualty Segment [Member]
Segment Reporting Information [Line Items]
Revenues																	767		711		693
Commercial Lines [Member] | Commercial Property Segment
Segment Reporting Information [Line Items]
Revenues																	545		497		489
Commercial Lines [Member] | Commercial Auto Segment
Segment Reporting Information [Line Items]
Revenues																	426		394		384
Commercial Lines [Member] | Workers' Compensation [Member]
Segment Reporting Information [Line Items]
Revenues																	344		318		311
Commercial Lines [Member] | Specialty Packages
Segment Reporting Information [Line Items]
Revenues																	151		138		149
Commercial Lines [Member] | Surety and executive risk
Segment Reporting Information [Line Items]
Revenues																	111		103		95
Commercial Lines [Member] | Machinery and Equipment
Segment Reporting Information [Line Items]
Revenues																	39		36		33
Commercial Lines [Member] | Commercial Lines Insurance Premiums [Member]
Segment Reporting Information [Line Items]
Revenues																	2,383		2,197		2,154
Commercial Lines [Member] | Other Fee Revenue
Segment Reporting Information [Line Items]
Revenues																	4		3		2
Personal Lines [Member]
Segment Reporting Information [Line Items]
Revenues																	870		763		723
Income (loss) before income taxes																	(43)		(182)		(55)
Personal Lines [Member] | Personal Auto
Segment Reporting Information [Line Items]
Revenues																	404		368		337
Personal Lines [Member] | Home Owner Segment
Segment Reporting Information [Line Items]
Revenues																	353		294		289
Personal Lines [Member] | Other Personal Lines [Member]
Segment Reporting Information [Line Items]
Revenues																	111		100		95
Personal Lines [Member] | Commercial Lines Insurance Premiums [Member]
Segment Reporting Information [Line Items]
Revenues																	868		762		721
Personal Lines [Member] | Other Fee Revenue
Segment Reporting Information [Line Items]
Revenues																	2		1		2
Excess And Surplus Lines Insurance [Member]
Segment Reporting Information [Line Items]
Revenues																	93		70		49
Income (loss) before income taxes																	(1)		6		(8)
Life Insurance Segment [Member]
Segment Reporting Information [Line Items]
Revenues																	178		165		158
Income (loss) before income taxes																	(3)		(3)		6
Identifiable assets	1,201								1,237								1,201		1,237
Total Life Insurance [Member]
Segment Reporting Information [Line Items]
Revenues																	179		167		159
Total Life Insurance [Member] | Separate Account Investment Management Fees [Member]
Segment Reporting Information [Line Items]
Revenues																	1		2		1
Investments Segment [Member]
Segment Reporting Information [Line Items]
Revenues																	573		595		677
Income (loss) before income taxes																	491		514		598
Identifiable assets	12,599								11,883								12,599		11,883
Investments Segment [Member] | Investment Income, Net Of Expenses [Member]
Segment Reporting Information [Line Items]
Revenues																	531		525		518
Investments Segment [Member] | Realized Investment Gains [Member]
Segment Reporting Information [Line Items]
Revenues																	42		70		159
Other [Member]
Segment Reporting Information [Line Items]
Revenues																	9		8		8
Income (loss) before income taxes																	(59)		(60)		(57)
Identifiable assets	$ 353								$ 243								$ 353		$ 243

[1] Revenues including realized investment gains and losses, which are integral to our financial results over the long term, may cause this value to fluctuate substantially because we have substantial discretion in the timing of investment sales. Also, applicable accounting standards require us to recognize gains and losses from certain changes in fair values of securities and embedded derivatives without actual realization of those gains and losses. We discuss realized investment gains for the past three years in Item 7, Investments Results of Operations, Page .