Segment Information (details) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Segment reporting information
Number of reportable business segments | item										3
Premiums										$ 23,874	$ 23,713	$ 22,637
Net investment income										2,379	2,787	2,716
Fee income										445	438	395
Other revenues										99	145	277
Income tax expense		$ 343	$ 379	$ 264	$ 315	$ 422	$ 339	$ 218	$ 418	1,301	1,397	1,272
Operating income (loss)										3,437	3,641	3,567
Net realized investment gains	[1]									3	79	166
Total revenues		6,674	6,794	6,706	6,626	6,783	6,886	6,785	6,708	26,800	27,162	26,191
Net realized investment gains, net of tax										2	51	106
Net income		$ 866	$ 928	$ 812	$ 833	$ 1,038	$ 919	$ 683	$ 1,052	3,439	3,692	3,673
Property casualty [Member]
Segment reporting information
Premiums										23,874	23,713	22,637
Net investment income										2,379	2,787	2,716
Domestic [Member]
Segment reporting information
Total revenues										25,112	25,091	25,138
International [Member]
Segment reporting information
Total revenues										1,688	2,071	1,053
Reportable Segments [Member]
Segment reporting information
Premiums										23,874	23,713	22,637
Net investment income										2,379	2,787	2,716
Fee income										445	438	395
Other revenues										93	145	283
Total operating revenues										26,791	27,083	26,031
Amortization and depreciation										4,696	4,738	4,685
Income tax expense										1,443	1,512	1,351
Operating income (loss)										3,692	3,898	3,815
Reportable Segments [Member] | Business and International Insurance [Member]
Segment reporting information
Premiums										14,521	14,512	13,332
Net investment income										1,824	2,156	2,087
Fee income										445	438	395
Other revenues										23	46	160
Total operating revenues										16,813	17,152	15,974
Amortization and depreciation										2,907	2,909	2,751
Income tax expense										769	798	758
Operating income (loss)										2,170	2,347	2,404
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums										12,650	12,334	12,084
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums										3,868	3,713	3,560
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums										1,925	1,901	1,904
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Commercial Property [Member]
Segment reporting information
Premiums										1,772	1,756	1,698
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | General Liability [Member]
Segment reporting information
Premiums										1,914	1,852	1,790
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums										3,132	3,070	3,093
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums										39	42	39
Reportable Segments [Member] | Business and International Insurance [Member] | International [Member]
Segment reporting information
Premiums										1,871	2,178	1,248
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums										2,085	2,076	1,981
Net investment income										223	252	260
Other revenues										22	19	20
Total operating revenues										2,330	2,347	2,261
Amortization and depreciation										467	482	473
Income tax expense										272	348	227
Operating income (loss)										633	727	573
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | General Liability [Member]
Segment reporting information
Premiums										955	963	891
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums										954	936	913
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Other [Member]
Segment reporting information
Premiums										176	177	177
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums										7,268	7,125	7,324
Net investment income										332	379	369
Other revenues										48	80	103
Total operating revenues										7,648	7,584	7,796
Amortization and depreciation										1,322	1,347	1,461
Income tax expense										402	366	366
Operating income (loss)										889	824	838
Reportable Segments [Member] | Personal Insurance [Member] | Automobile [Member]
Segment reporting information
Premiums										3,512	3,316	3,431
Reportable Segments [Member] | Personal Insurance [Member] | Homeowners and other [Member]
Segment reporting information
Premiums										3,756	3,809	3,893
Other [Member]
Segment reporting information
Other revenues										6		(6)
Operating income (loss)										(255)	(257)	(248)
After-tax interest expense										$ 242	$ 240	$ 235

[1] Total other-than-temporary impairment (OTTI) losses were $(54) million, $(22) million and $(10) million for the years ended December 31, 2015, 2014 and 2013, respectively. Of total OTTI, credit losses of $(52) million, $(26) million and $(15) million for the years ended December 31, 2015, 2014 and 2013, respectively, were recognized in net realized investment gains. In addition, unrealized gains (losses) from other changes in total OTTI of $(2) million, $4 million and $5 million for the years ended December 31, 2015, 2014 and 2013, respectively, were recognized in other comprehensive income (loss) as part of changes in net unrealized gains on investment securities having credit losses recognized in the consolidated statement of income.