Segment Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($) segment	Mar. 31, 2020 USD ($)
Segment reporting information
Number of reportable business segments | segment	3
Premiums	$ 7,386	$ 7,229
Net investment income	701	611
Fee income	101	108
Other revenues	81	58
Net realized investment gains (losses)	44	(98)
Total revenues	8,313	7,908
Core income (loss)	699	676
Net realized investment gains (losses), net of tax	34	(76)
Net income	733	600
Reportable Segments [Member]
Segment reporting information
Premiums	7,386	7,229
Net investment income	701	611
Fee income	101	108
Other revenues	81	58
Total segment revenues	8,269	8,006
Core income (loss)	768	747
Reportable Segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums	3,799	3,864
Net investment income	523	453
Fee income	95	102
Other revenues	53	31
Total segment revenues	4,470	4,450
Core income (loss)	317	289
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	3,528	3,590
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums	811	899
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	695	684
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums	531	508
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	607	601
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums	870	886
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	14	12
Reportable Segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums	271	274
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums	743	667
Net investment income	59	55
Fee income	0	0
Other revenues	5	5
Total segment revenues	807	727
Core income (loss)	137	122
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	653	602
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	331	287
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums	266	257
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	56	58
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums	90	65
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums	2,844	2,698
Net investment income	119	103
Fee income	6	6
Other revenues	23	22
Total segment revenues	2,992	2,829
Core income (loss)	314	336
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	2,678	2,527
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	1,370	1,351
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums	1,308	1,176
Reportable Segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums	166	171
Other [Member]
Segment reporting information
Other revenues	0	0
Core income (loss)	(69)	(71)
After-tax interest expense	$ 65	$ 66