Segment Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($)
Segment reporting information
Number of reportable business segments | segment			3
Premiums	$ 7,380	$ 7,179	$ 21,564	$ 21,022
Net investment income	671	622	1,550	1,852
Fee income	101	121	323	346
Other revenues	86	68	195	197
Net realized investment gains (losses)	37	23	(48)	101
Total revenues	8,275	8,013	23,584	23,518
Core income (loss)	798	378	1,424	1,670
Net realized investment gains (losses), net of tax	29	18	(37)	79
Net income	827	396	1,387	1,749
Reportable Segments [Member]
Segment reporting information
Premiums	7,380	7,179	21,564	21,022
Net investment income	671	622	1,550	1,852
Fee income	101	121	323	346
Other revenues	86	68	195	196
Total segment revenues	8,238	7,990	23,632	23,416
Core income (loss)	872	449	1,643	1,892
Reportable Segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums	3,841	3,882	11,440	11,407
Net investment income	498	457	1,131	1,365
Fee income	95	114	305	329
Other revenues	58	39	125	112
Total segment revenues	4,492	4,492	13,001	13,213
Core income (loss)	365	179	596	944
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	3,582	3,615	10,654	10,610
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums	851	969	2,568	2,891
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	698	672	2,058	1,947
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums	522	496	1,544	1,429
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	605	594	1,786	1,746
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums	891	872	2,660	2,568
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	15	12	38	29
Reportable Segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums	259	267	786	797
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums	723	653	2,083	1,891
Net investment income	58	59	155	173
Fee income	0	0	0	0
Other revenues	7	7	17	19
Total segment revenues	788	719	2,255	2,083
Core income (loss)	115	139	309	451
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	647	595	1,876	1,727
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	310	276	893	799
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums	277	265	805	769
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	60	54	178	159
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums	76	58	207	164
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums	2,816	2,644	8,041	7,724
Net investment income	115	106	264	314
Fee income	6	7	18	17
Other revenues	21	22	53	65
Total segment revenues	2,958	2,779	8,376	8,120
Core income (loss)	392	131	738	497
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	2,653	2,463	7,555	7,198
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	1,391	1,344	3,896	3,962
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums	1,262	1,119	3,659	3,236
Reportable Segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums	163	181	486	526
Other [Member]
Segment reporting information
Other revenues	0	0	0	1
Core income (loss)	(74)	(71)	(219)	(222)
After-tax interest expense	$ 69	$ 66	$ 202	$ 206