Segment Information (Details) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment reporting information
Number of reportable segments | segment										3
Premiums										$ 28,272	$ 27,059	$ 25,683
Net investment income										2,468	2,474	2,397
Fee income										459	432	447
Other revenues										269	203	159
Income tax expense (benefit)		$ 201	$ 36	$ 108	$ 171	$ 125	$ 97	$ 107	$ 109	516	438	674
Net realized investment gains	[1]									113	114	216
Total revenues		8,063	8,013	7,834	7,671	7,796	7,723	7,477	7,286	31,581	30,282	28,902
Core income (loss)										2,537	2,430	2,043
Net realized investment gains, net of tax										85	93	142
Impact of Tax Cuts and Jobs Act of 2017 at enactment												(129)
Net income		$ 873	$ 396	$ 557	$ 796	$ 621	$ 709	$ 524	$ 669	2,622	2,523	2,056
Domestic [Member]
Segment reporting information
Total revenues										29,638	28,418	27,253
International [Member]
Segment reporting information
Total revenues										1,943	1,864	1,649
Reportable segments [Member]
Segment reporting information
Premiums										28,272	27,059	25,683
Net investment income										2,468	2,474	2,397
Fee income										459	432	447
Other revenues										268	201	153
Total segment revenues										31,467	30,166	28,680
Amortization and depreciation										5,357	5,177	4,972
Income tax expense (benefit)										569	499	612
Core income (loss)										2,834	2,728	2,297
Reportable segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums										15,300	14,722	14,146
Net investment income										1,816	1,833	1,786
Fee income										437	412	430
Other revenues										155	112	69
Total segment revenues										17,708	17,079	16,431
Amortization and depreciation										3,037	2,943	2,852
Income tax expense (benefit)										223	259	448
Core income (loss)										1,392	1,638	1,613
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums										14,233	13,657	13,143
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums										3,829	3,899	3,962
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums										2,632	2,388	2,132
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums										1,937	1,828	1,775
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums										2,342	2,181	2,047
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums										3,453	3,333	3,198
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums										40	28	29
Reportable segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums										1,067	1,065	1,003
Reportable segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums										2,565	2,420	2,307
Net investment income										233	233	228
Other revenues										26	23	24
Total segment revenues										2,824	2,676	2,559
Amortization and depreciation										533	515	493
Income tax expense (benefit)										151	198	208
Core income (loss)										618	793	556
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums										2,334	2,216	2,126
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums										1,082	1,004	962
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums										1,036	1,017	977
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums										216	195	187
Reportable segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums										231	204	181
Reportable segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums										10,407	9,917	9,230
Net investment income										419	408	383
Fee income										22	20	17
Other revenues										87	66	60
Total segment revenues										10,935	10,411	9,690
Amortization and depreciation										1,787	1,719	1,627
Income tax expense (benefit)										195	42	(44)
Core income (loss)										824	297	128
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums										9,704	9,232	8,598
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums										5,311	5,097	4,655
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums										4,393	4,135	3,943
Reportable segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums										703	685	632
Other [Member]
Segment reporting information
Other revenues										1	2	6
Core income (loss)										(297)	(298)	(254)
After-tax interest expense										$ 272	$ 278	$ 240

[1] Total other-than-temporary impairment (OTTI) losses were $(3) million, $(1) million and $(13) million for the years ended December 31, 2019, 2018 and 2017, respectively.  Of total OTTI, credit losses of $(4) million, $(1) million and $(14) million for the years ended December 31, 2019, 2018 and 2017, respectively, were recognized in net realized investment gains.  In addition, unrealized gains (losses) from other changes in total OTTI of $1 million, $0 million and $1 million for the years ended December 31, 2019, 2018 and 2017, 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.