Segment Information (Details) $ in Millions	3 Months Ended
Mar. 31, 2020 USD ($) segment	Mar. 31, 2019 USD ($)
Segment reporting information
Number of reportable business segments \| segment	3
Premiums	$ 7,229	$ 6,855
Net investment income	611	582
Fee income	108	109
Other revenues	58	72
Net realized investment gains (losses)	(98)	53
Total revenues	7,908	7,671
Core income (loss)	676	755
Net realized investment gains (losses), net of tax	(76)	41
Net income	600	796
Reportable Segments [Member]
Segment reporting information
Premiums	7,229	6,855
Net investment income	611	582
Fee income	108	109
Other revenues	58	71
Total segment revenues	8,006	7,617
Core income (loss)	747	830
Reportable Segments [Member] \| Business Insurance [Member]
Segment reporting information
Premiums	3,864	3,742
Net investment income	453	427
Fee income	102	104
Other revenues	31	43
Total segment revenues	4,450	4,316
Core income (loss)	289	414
Reportable Segments [Member] \| Business Insurance [Member] \| Domestic [Member]
Segment reporting information
Premiums	3,590	3,475
Reportable Segments [Member] \| Business Insurance [Member] \| Domestic [Member] \| Workers' compensation [Member]
Segment reporting information
Premiums	899	972
Reportable Segments [Member] \| Business Insurance [Member] \| Domestic [Member] \| Automobile [Member]
Segment reporting information
Premiums	684	628
Reportable Segments [Member] \| Business Insurance [Member] \| Domestic [Member] \| Commercial property [Member]
Segment reporting information
Premiums	508	460
Reportable Segments [Member] \| Business Insurance [Member] \| Domestic [Member] \| General liability [Member]
Segment reporting information
Premiums	601	567
Reportable Segments [Member] \| Business Insurance [Member] \| Domestic [Member] \| Commercial multi-peril [Member]
Segment reporting information
Premiums	886	840
Reportable Segments [Member] \| Business Insurance [Member] \| Domestic [Member] \| Other [Member]
Segment reporting information
Premiums	12	8
Reportable Segments [Member] \| Business Insurance [Member] \| International [Member]
Segment reporting information
Premiums	274	267
Reportable Segments [Member] \| Bond & Specialty Insurance [Member]
Segment reporting information
Premiums	667	606
Net investment income	55	56
Fee income	0	0
Other revenues	5	6
Total segment revenues	727	668
Core income (loss)	122	138
Reportable Segments [Member] \| Bond & Specialty Insurance [Member] \| Domestic [Member]
Segment reporting information
Premiums	602	555
Reportable Segments [Member] \| Bond & Specialty Insurance [Member] \| Domestic [Member] \| General liability [Member]
Segment reporting information
Premiums	287	257
Reportable Segments [Member] \| Bond & Specialty Insurance [Member] \| Domestic [Member] \| Fidelity and surety [Member]
Segment reporting information
Premiums	257	246
Reportable Segments [Member] \| Bond & Specialty Insurance [Member] \| Domestic [Member] \| Other [Member]
Segment reporting information
Premiums	58	52
Reportable Segments [Member] \| Bond & Specialty Insurance [Member] \| International [Member]
Segment reporting information
Premiums	65	51
Reportable Segments [Member] \| Personal Insurance [Member]
Segment reporting information
Premiums	2,698	2,507
Net investment income	103	99
Fee income	6	5
Other revenues	22	22
Total segment revenues	2,829	2,633
Core income (loss)	336	278
Reportable Segments [Member] \| Personal Insurance [Member] \| Domestic [Member]
Segment reporting information
Premiums	2,527	2,336
Reportable Segments [Member] \| Personal Insurance [Member] \| Domestic [Member] \| Automobile [Member]
Segment reporting information
Premiums	1,351	1,297
Reportable Segments [Member] \| Personal Insurance [Member] \| Domestic [Member] \| Homeowners and Other [Member]
Segment reporting information
Premiums	1,176	1,039
Reportable Segments [Member] \| Personal Insurance [Member] \| International [Member]
Segment reporting information
Premiums	171	171
Other [Member]
Segment reporting information
Other revenues	0	1
Core income (loss)	(71)	(75)
After-tax interest expense	$ 66	$ 70

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