Segment Information (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($)
Segment reporting information
Number of reportable business segments | segment			3
Premiums	$ 6,955	$ 6,988	$ 14,184	$ 13,843
Net investment income	268	648	879	1,230
Fee income	114	116	222	225
Other revenues	51	57	109	129
Net realized investment gains (losses)	13	25	(85)	78
Total revenues	7,401	7,834	15,309	15,505
Core income (loss)	(50)	537	626	1,292
Net realized investment gains (losses), net of tax	10	20	(66)	61
Net income (loss)	(40)	557	560	1,353
Reportable Segments [Member]
Segment reporting information
Premiums	6,955	6,988	14,184	13,843
Net investment income	268	648	879	1,230
Fee income	114	116	222	225
Other revenues	51	57	109	128
Total segment revenues	7,388	7,809	15,394	15,426
Core income (loss)	24	613	771	1,443
Reportable Segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums	3,735	3,783	7,599	7,525
Net investment income	180	481	633	908
Fee income	108	111	210	215
Other revenues	36	30	67	73
Total segment revenues	4,059	4,405	8,509	8,721
Core income (loss)	(58)	351	231	765
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	3,482	3,520	7,072	6,995
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums	818	950	1,717	1,922
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	676	647	1,360	1,275
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums	514	473	1,022	933
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	580	585	1,181	1,152
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums	883	856	1,769	1,696
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	11	9	23	17
Reportable Segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums	253	263	527	530
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums	693	632	1,360	1,238
Net investment income	42	58	97	114
Fee income	0	0	0	0
Other revenues	5	6	10	12
Total segment revenues	740	696	1,467	1,364
Core income (loss)	72	174	194	312
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	627	577	1,229	1,132
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums	296	266	583	523
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums	271	258	528	504
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums	60	53	118	105
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums	66	55	131	106
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums	2,527	2,573	5,225	5,080
Net investment income	46	109	149	208
Fee income	6	5	12	10
Other revenues	10	21	32	43
Total segment revenues	2,589	2,708	5,418	5,341
Core income (loss)	10	88	346	366
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums	2,375	2,399	4,902	4,735
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums	1,154	1,321	2,505	2,618
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums	1,221	1,078	2,397	2,117
Reportable Segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums	152	174	323	345
Other [Member]
Segment reporting information
Other revenues	0	0	0	1
Core income (loss)	(74)	(76)	(145)	(151)
After-tax interest expense	$ 67	$ 70	$ 133	$ 140