Segment Information (Details) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment reporting information
Number of reportable segments | segment										3
Premiums										$ 27,059	$ 25,683	$ 24,534
Net investment income										2,474	2,397	2,302
Fee income										432	447	458
Other revenues										203	159	263
Income tax expense (benefit)		$ 125	$ 97	$ 107	$ 109	$ 309	$ 27	$ 195	$ 143	438	674	1,039
Net realized investment gains	[1]									114	216	68
Total revenues		7,796	7,723	7,477	7,286	7,451	7,325	7,184	6,942	30,282	28,902	27,625
Core income (loss)										2,430	2,043	2,967
Net realized investment gains, net of tax										93	142	47
Impact of Tax Cuts and Jobs Act of 2017 at enactment											(129)
Net income		$ 621	$ 709	$ 524	$ 669	$ 551	$ 293	$ 595	$ 617	2,523	2,056	3,014
Domestic [Member]
Segment reporting information
Total revenues										28,418	27,253	25,904
International [Member]
Segment reporting information
Total revenues										1,864	1,649	1,721
Reportable segments [Member]
Segment reporting information
Premiums										27,059	25,683	24,534
Net investment income										2,474	2,397	2,302
Fee income										432	447	458
Other revenues										201	153	252
Total segment revenues										30,166	28,680	27,546
Amortization and depreciation										5,177	4,972	4,804
Income tax expense (benefit)										499	612	1,157
Core income (loss)										2,728	2,297	3,211
Reportable segments [Member] | Business Insurance [Member]
Segment reporting information
Premiums										14,722	14,146	13,855
Net investment income										1,833	1,786	1,701
Fee income										412	430	442
Other revenues										112	69	168
Total segment revenues										17,079	16,431	16,166
Amortization and depreciation										2,943	2,852	2,783
Income tax expense (benefit)										259	448	656
Core income (loss)										1,638	1,613	1,982
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums										13,657	13,143	12,904
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums										3,899	3,962	3,969
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums										2,388	2,132	2,010
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial property [Member]
Segment reporting information
Premiums										1,828	1,775	1,769
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums										2,181	2,047	1,977
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums										3,333	3,198	3,148
Reportable segments [Member] | Business Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums										28	29	31
Reportable segments [Member] | Business Insurance [Member] | International [Member]
Segment reporting information
Premiums										1,065	1,003	951
Reportable segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums										2,420	2,307	2,260
Net investment income										233	228	239
Other revenues										23	24	21
Total segment revenues										2,676	2,559	2,520
Amortization and depreciation										515	493	491
Income tax expense (benefit)										198	208	309
Core income (loss)										793	556	712
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums										2,216	2,126	2,088
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | General liability [Member]
Segment reporting information
Premiums										1,004	962	946
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums										1,017	977	962
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums										195	187	180
Reportable segments [Member] | Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Premiums										204	181	172
Reportable segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums										9,917	9,230	8,419
Net investment income										408	383	362
Fee income										20	17	16
Other revenues										66	60	63
Total segment revenues										10,411	9,690	8,860
Amortization and depreciation										1,719	1,627	1,530
Income tax expense (benefit)										42	(44)	192
Core income (loss)										297	128	517
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums										9,232	8,598	7,826
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums										5,097	4,655	4,013
Reportable segments [Member] | Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Premiums										4,135	3,943	3,813
Reportable segments [Member] | Personal Insurance [Member] | International [Member]
Segment reporting information
Premiums										685	632	593
Other [Member]
Segment reporting information
Other revenues										2	6	11
Core income (loss)										(298)	(254)	(244)
After-tax interest expense										$ 278	$ 240	$ 236

[1] Total other-than-temporary impairment (OTTI) losses were $(1) million, $(13) million and $(40) million for the years ended December 31, 2018, 2017 and 2016, respectively. Of total OTTI, credit losses of $(1) million, $(14) million and $(29) million for the years ended December 31, 2018, 2017 and 2016, respectively, were recognized in net realized investment gains. In addition, unrealized gains (losses) from other changes in total OTTI of $0 million, $1 million and $(11) million for the years ended December 31, 2018, 2017 and 2016, 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.