Segment Information (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Segment reporting information
Premiums		$ 6,855	$ 6,537
Net investment income		582	603
Fee income		109	103
Other revenues		72	54
Net realized investment gains (losses)	[1]	53	(11)
Total revenues		7,671	7,286
Core income (loss)		755	678
Net realized investment gains (losses), net of tax		41	(9)
Net income		796	669
Reportable Segments [Member]
Segment reporting information
Premiums		6,855	6,537
Net investment income		582	603
Fee income		109	103
Other revenues		71	54
Total segment revenues		7,617	7,297
Core income (loss)		830	754
Reportable Segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums		3,742	3,568
Net investment income		427	446
Fee income		104	99
Other revenues		43	31
Total segment revenues		4,316	4,144
Core income (loss)		414	452
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums		3,475	3,304
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums		972	971
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums		628	562
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums		460	438
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums		567	521
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums		840	805
Reportable Segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums		8	7
Reportable Segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums		267	264
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums		606	582
Net investment income		56	58
Other revenues		6	6
Total segment revenues		668	646
Core income (loss)		138	173
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums		555	535
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums		257	242
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums		246	246
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums		52	47
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums		51	47
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums		2,507	2,387
Net investment income		99	99
Fee income		5	4
Other revenues		22	17
Total segment revenues		2,633	2,507
Core income (loss)		278	129
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums		2,336	2,220
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums		1,297	1,225
Reportable Segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums		1,039	995
Reportable Segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums		171	167
Other [Member]
Segment reporting information
Other revenues		1
Core income (loss)		(75)	(76)
After-tax interest expense		$ 70	$ 70

[1] Total other-than-temporary impairment (OTTI) gains (losses) were $(1) million and $0 for the three months ended March 31, 2019 and 2018, respectively. Of total OTTI, credit losses of $(1) million and $0 for the three months ended March 31, 2019 and 2018, respectively, were recognized in net realized investment gains (losses). In addition, unrealized gains (losses) from other changes in total OTTI of $0 for each of the three months ended March 31, 2019 and 2018, respectively, were recognized in other comprehensive income (loss) as part of changes in net unrealized gains (losses) on investment securities having credit losses recognized in the consolidated statement of income.