Segment Information (details) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) segments	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment reporting information
Number of reportable business segments | segments										3
Premiums										$ 25,683	$ 24,534	$ 23,874
Net investment income										2,397	2,302	2,379
Fee income										447	458	460
Other revenues										159	263	99
Income tax expense		$ 309	$ 27	$ 195	$ 143	$ 359	$ 231	$ 223	$ 226	674	1,039	1,301
Core income (loss)										2,043	2,967	3,437
Net realized investment gains	[1]									216	68	3
Total revenues		7,451	7,325	7,184	6,942	7,193	6,961	6,785	6,686	28,902	27,625	26,815
Net realized investment gains, net of tax										142	47	2
Impact of Tax Cuts and Jobs Act of 2017 at enactment		(129)								(129)
Net income		$ 551	$ 293	$ 595	$ 617	$ 943	$ 716	$ 664	$ 691	2,056	3,014	3,439
Property casualty [Member]
Segment reporting information
Premiums										25,683	24,534	23,874
Net investment income										2,397	2,302	2,379
Domestic [Member]
Segment reporting information
Total revenues										27,253	25,904	25,127
International [Member]
Segment reporting information
Total revenues										1,649	1,721	1,688
Reportable Segments [Member]
Segment reporting information
Premiums										25,683	24,534	23,874
Net investment income										2,397	2,302	2,379
Fee income										447	458	460
Other revenues										153	252	93
Total segment revenues										28,680	27,546	26,806
Amortization and depreciation										4,972	4,804	4,696
Income tax expense										612	1,157	1,443
Core income (loss)										2,297	3,211	3,692
Reportable Segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums										14,146	13,855	13,698
Net investment income										1,786	1,701	1,757
Fee income										430	442	445
Other revenues										69	168	17
Total segment revenues										16,431	16,166	15,917
Amortization and depreciation										2,852	2,783	2,725
Income tax expense										448	656	744
Core income (loss)										1,613	1,982	2,077
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums										13,143	12,904	12,625
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums										3,962	3,969	3,867
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums										2,132	2,010	1,922
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums										1,775	1,769	1,766
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums										2,047	1,977	1,898
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums										3,198	3,148	3,133
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums										29	31	39
Reportable Segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums										1,003	951	1,073
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums										2,307	2,260	2,267
Net investment income										228	239	254
Other revenues										24	21	22
Total segment revenues										2,559	2,520	2,543
Amortization and depreciation										493	491	501
Income tax expense										208	309	283
Core income (loss)										556	712	683
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums										2,126	2,088	2,085
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums										962	946	955
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums										977	962	954
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums										187	180	176
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums										181	172	182
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums										9,230	8,419	7,909
Net investment income										383	362	368
Fee income										17	16	15
Other revenues										60	63	54
Total segment revenues										9,690	8,860	8,346
Amortization and depreciation										1,627	1,530	1,470
Income tax expense										(44)	192	416
Core income (loss)										128	517	932
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums										8,598	7,826	7,268
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums										4,655	4,013	3,512
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums										3,943	3,813	3,756
Reportable Segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums										632	593	641
Other [Member]
Segment reporting information
Other revenues										6	11	6
Core income (loss)										(254)	(244)	(255)
After-tax interest expense										$ 240	$ 236	$ 242

[1] Total other-than-temporary impairment (OTTI) losses were $(13) million, $(40) million and $(54) million for the years ended December 31, 2017, 2016 and 2015, respectively. Of total OTTI, credit losses of $(14) million, $(29) million and $(52) million for the years ended December 31, 2017, 2016 and 2015, respectively, were recognized in net realized investment gains. In addition, unrealized gains (losses) from other changes in total OTTI of $1 million, $(11) million and $(2) million for the years ended December 31, 2017, 2016 and 2015, 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 statements of income.