Segment Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) segment	Sep. 30, 2020 USD ($)
Segment reporting information
Number of reportable business segments | segment			3
Premiums	$ 7,829	$ 7,380	$ 22,831	$ 21,564
Net investment income	771	671	2,290	1,550
Fee income	97	101	302	323
Other revenues	100	86	269	195
Net realized investment gains (losses)	8	37	113	(48)
Total revenues	8,805	8,275	25,805	23,584
Core income (loss)	655	798	2,233	1,424
Net realized investment gains (losses), net of tax	7	29	88	(37)
Impact of changes in tax laws and/or tax rates	0	0	8	0
Net income	662	827	2,329	1,387
Reportable Segments [Member]
Segment reporting information
Premiums	7,829	7,380	22,831	21,564
Net investment income	771	671	2,290	1,550
Fee income	97	101	302	323
Other revenues	100	86	269	195
Total segment revenues	8,797	8,238	25,692	23,632
Core income (loss)	730	872	2,449	1,643
Reportable Segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums	3,970	3,841	11,649	11,440
Net investment income	575	498	1,713	1,131
Fee income	90	95	282	305
Other revenues	69	58	179	125
Total segment revenues	4,704	4,492	13,823	13,001
Core income (loss)	558	365	1,518	596
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	3,707	3,582	10,841	10,654
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums	789	851	2,410	2,568
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	718	698	2,122	2,058
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums	582	522	1,669	1,544
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	662	605	1,887	1,786
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums	939	891	2,708	2,660
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	17	15	45	38
Reportable Segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums	263	259	808	786
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums	806	723	2,325	2,083
Net investment income	63	58	186	155
Fee income	0	0	0	0
Other revenues	5	7	17	17
Total segment revenues	874	788	2,528	2,255
Core income (loss)	174	115	498	309
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	696	647	2,027	1,876
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	365	310	1,043	893
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums	277	277	819	805
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	54	60	165	178
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums	110	76	298	207
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums	3,053	2,816	8,857	8,041
Net investment income	133	115	391	264
Fee income	7	6	20	18
Other revenues	26	21	73	53
Total segment revenues	3,219	2,958	9,341	8,376
Core income (loss)	(2)	392	433	738
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	2,882	2,653	8,342	7,555
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	1,445	1,391	4,220	3,896
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums	1,437	1,262	4,122	3,659
Reportable Segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums	171	163	515	486
Other [Member]
Segment reporting information
Core income (loss)	(75)	(74)	(216)	(219)
After-tax interest expense	$ 69	$ 69	$ 199	$ 202