Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segment reporting information
Number of reportable business segments | segment									3
Premiums									$ 29,044	$ 28,272	$ 27,059
Net investment income									2,227	2,468	2,474
Fee income									429	459	432
Other revenues									279	269	203
Income tax expense	$ 306	$ 159	$ (45)	$ 120	$ 201	$ 36	$ 108	$ 171	540	516	438
Net realized investment gains									2	113	114
Total revenues	8,397	8,275	7,401	7,908	8,063	8,013	7,834	7,671	31,981	31,581	30,282
Core income (loss)									2,686	2,537	2,430
Net realized investment gains, net of tax									11	85	93
Net income	$ 1,310	$ 827	$ (40)	$ 600	$ 873	$ 396	$ 557	$ 796	2,697	2,622	2,523
Domestic [Member]
Segment reporting information
Total revenues									30,123	29,638	28,418
International [Member]
Segment reporting information
Total revenues									1,858	1,943	1,864
Reportable segments [Member]
Segment reporting information
Premiums									29,044	28,272	27,059
Net investment income									2,227	2,468	2,474
Fee income									429	459	432
Other revenues									279	268	201
Total segment revenues									31,979	31,467	30,166
Amortization and depreciation									5,556	5,357	5,177
Income tax expense									628	569	499
Core income (loss)									2,977	2,834	2,728
Reportable segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums									15,294	15,300	14,722
Net investment income									1,633	1,816	1,833
Fee income									405	437	412
Other revenues									176	155	112
Total segment revenues									17,508	17,708	17,079
Amortization and depreciation									3,069	3,037	2,943
Income tax expense									213	223	259
Core income (loss)									1,309	1,392	1,638
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums									14,233	14,233	13,657
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums									3,378	3,829	3,899
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums									2,761	2,632	2,388
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums									2,087	1,937	1,828
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums									2,401	2,342	2,181
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums									3,552	3,453	3,333
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums									54	40	28
Reportable segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums									1,061	1,067	1,065
Reportable segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums									2,823	2,565	2,420
Net investment income									213	233	233
Fee income									0	0	0
Other revenues									27	26	23
Total segment revenues									3,063	2,824	2,676
Amortization and depreciation									579	533	515
Income tax expense									107	151	198
Core income (loss)									473	618	793
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums									2,531	2,334	2,216
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums									1,219	1,082	1,004
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums									1,075	1,036	1,017
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums									237	216	195
Reportable segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums									292	231	204
Reportable segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums									10,927	10,407	9,917
Net investment income									381	419	408
Fee income									24	22	20
Other revenues									76	87	66
Total segment revenues									11,408	10,935	10,411
Amortization and depreciation									1,908	1,787	1,719
Income tax expense									308	195	42
Core income (loss)									1,195	824	297
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums									10,268	9,704	9,232
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums									5,280	5,311	5,097
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums									4,988	4,393	4,135
Reportable segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums									659	703	685
Other [Member]
Segment reporting information
Other revenues									0	1	2
Core income (loss)									(291)	(297)	(298)
After-tax interest expense									$ 268	$ 272	$ 278