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 item		Dec. 31, 2011		Dec. 31, 2010
Segment reporting information
Premiums									$ 22,357		$ 22,090		$ 21,432
Net investment income									2,889		2,879		3,059
Fee income									323		296		287
Other revenues									120		126		70
Income tax expense (benefit)	7	306	109	271	160	(1)	(363)	130	693		(74)		1,090
Operating income (loss)									2,441		1,390		3,043
Net written premiums									22,447		22,187		21,635
Segment Information disclosure
Number of reportable business segments									3
Net realized investment gains									51	[1]	55	[1]	264	[1]
Total revenues	6,477	6,512	6,359	6,392	6,373	6,407	6,388	6,278	25,740		25,446		25,112
Operating income (loss)									2,441		1,390		3,043
Net realized investment gains, net of tax									32		36		173
Net income	304	864	499	806	618	333	(364)	839	2,473		1,426		3,216
Revenues of the Company's operations based on location, U.S.									24,827		24,408		24,049
Revenues of the Company's operations based on location, non-U.S.									913		1,038		1,063
Total reportable segments [Member]
Segment reporting information
Premiums									22,357		22,090		21,432
Net investment income									2,889		2,879		3,059
Fee income									323		296		287
Other revenues									132		127		130
Total operating revenues									25,701		25,392		24,908
Amortization and depreciation									4,726		4,668		4,608
Income tax expense (benefit)									826		71		1,156
Operating income (loss)									2,702		1,669		3,361
Net written premiums									22,447		22,187		21,635
Segment Information disclosure
Operating income (loss)									2,702		1,669		3,361
Business Insurance [Member]
Segment reporting information
Premiums									11,691		11,327		10,766
Net investment income									2,090		2,041		2,156
Fee income									322		295		285
Other revenues									40		31		28
Total operating revenues									14,143		13,694		13,235
Amortization and depreciation									2,393		2,313		2,253
Income tax expense (benefit)									539		134		777
Operating income (loss)									1,843		1,354		2,301
Net written premiums									11,872		11,340		10,857
Segment Information disclosure
Operating income (loss)									1,843		1,354		2,301
Total Business Insurance Core [Member]
Segment reporting information
Net written premiums									11,873		11,330		10,844
Select Accounts [Member]
Segment reporting information
Net written premiums									2,775		2,784		2,718
Commercial Accounts [Member]
Segment reporting information
Net written premiums									3,101		2,890		2,576
National Accounts [Member]
Segment reporting information
Net written premiums									907		782		806
Industry-Focused Underwriting [Member]
Segment reporting information
Net written premiums									2,554		2,407		2,299
Target Risk Underwriting [Member]
Segment reporting information
Net written premiums									1,666		1,587		1,573
Specialized Distribution [Member]
Segment reporting information
Net written premiums									870		880		872
Workers' compensation [Member]
Segment reporting information
Premiums									3,222		2,899		2,489
Commercial automobile [Member]
Segment reporting information
Premiums									1,943		1,940		1,912
Commercial property [Member]
Segment reporting information
Premiums									1,621		1,607		1,669
Business Insurance, General liability [Member]
Segment reporting information
Premiums									1,757		1,738		1,739
Commercial multi-peril [Member]
Segment reporting information
Premiums									3,113		3,126		2,958
Business Insurance, Other [Member]
Segment reporting information
Premiums									35		17		(1)
Net written premiums									(1)		10		13
Financial, Professional & International Insurance [Member]
Segment reporting information
Premiums									3,045		3,174		3,317
Net investment income									395		414		439
Fee income									1		1		2
Other revenues									26		26		27
Total operating revenues									3,467		3,615		3,785
Amortization and depreciation									731		740		754
Income tax expense (benefit)									255		230		245
Operating income (loss)									642		647		620
Net written premiums									2,981		3,102		3,211
Segment Information disclosure
Operating income (loss)									642		647		620
Bond and Financial Products [Member]
Segment reporting information
Net written premiums									1,924		1,953		1,981
Fidelity and surety [Member]
Segment reporting information
Premiums									939		970		1,020
Financial, Professional & International Insurance, General liability [Member]
Segment reporting information
Premiums									850		832		866
International [Member]
Segment reporting information
Premiums									1,088		1,218		1,276
Net written premiums									1,057		1,149		1,230
Financial, Professional & International Insurance, Other [Member]
Segment reporting information
Premiums									168		154		155
Personal Insurance [Member]
Segment reporting information
Premiums									7,621		7,589		7,349
Net investment income									404		424		464
Other revenues									66		70		75
Total operating revenues									8,091		8,083		7,888
Amortization and depreciation									1,602		1,615		1,601
Income tax expense (benefit)									32		(293)		134
Operating income (loss)									217		(332)		440
Net written premiums									7,594		7,745		7,567
Segment Information disclosure
Operating income (loss)									217		(332)		440
Automobile [Member]
Segment reporting information
Premiums									3,665		3,720		3,693
Net written premiums									3,642		3,788		3,772
Homeowners and other [Member]
Segment reporting information
Premiums									3,956		3,869		3,656
Net written premiums									3,952		3,957		3,795
Interest Expense and Other [Member]
Segment reporting information
Other revenues									(12)		(1)		(60)
Operating income (loss)									(261)		(279)		(318)
After-tax interest expense									246		251		252
After-tax expenses related to the purchase and retirement of a significant portion of the 6.25% fixed-to-floating rate junior subordinated debentures													39
Segment Information disclosure
Operating income (loss)									$ (261)		$ (279)		$ (318)

[1] Total other-than-temporary impairment (OTTI) gains were $27 million, $30 million and $7 million for the years ended December 31, 2012, 2011 and 2010, respectively. Of total OTTI, credit losses of $(15) million, $(25) million and $(26) million for the years ended December 31, 2012, 2011 and 2010, respectively, were recognized in net realized investment gains. In addition, unrealized gains from other changes in total OTTI of $42 million, $55 million and $33 million for the years ended December 31, 2012, 2011 and 2010, respectively, were recognized in other comprehensive income as part of changes in net unrealized gains on investment securities having credit losses recognized in the consolidated statement of income.