Schedule III (details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule III
Deferred acquisition costs	$ 2,025	$ 1,923	$ 1,849
Claims and claim adjustment expense reserves	49,650	47,949	48,295
Unearned premiums	12,915	12,329	11,971
Earned premiums	25,683	24,534	23,874
Net investment income	2,397	2,302	2,379
Claims and claim adjustment expenses	17,467	15,070	13,723
Amortization of deferred acquisition costs	4,166	3,985	3,885
Other operating expenses	4,539	4,517	4,467
Net written premiums	26,219	24,958	24,121
Business Insurance [Member]
Schedule III
Net written premiums	14,270	13,900	13,774
Bond & Specialty Insurance [Member]
Schedule III
Net written premiums	2,359	2,271	2,273
Personal Insurance [Member]
Schedule III
Net written premiums	9,590	8,787	8,074
Reportable Segments [Member]
Schedule III
Deferred acquisition costs	2,025	1,923	1,849
Claims and claim adjustment expense reserves	49,633	47,929	48,272
Unearned premiums	12,915	12,329	11,971
Earned premiums	25,683	24,534	23,874
Net investment income	2,397	2,302	2,379
Claims and claim adjustment expenses	17,467	15,070	13,723
Amortization of deferred acquisition costs	4,166	3,985	3,885
Other operating expenses	4,138	4,123	4,063
Net written premiums	26,219	24,958	24,121
Reportable Segments [Member] | Business Insurance [Member]
Schedule III
Deferred acquisition costs	1,060	1,026	1,008
Claims and claim adjustment expense reserves	40,352	39,555	39,983
Unearned premiums	6,857	6,725	6,741
Earned premiums	14,146	13,855	13,698
Net investment income	1,786	1,701	1,757
Claims and claim adjustment expenses	9,521	8,753	8,409
Amortization of deferred acquisition costs	2,286	2,221	2,182
Other operating expenses	2,563	2,554	2,505
Net written premiums	14,270	13,900	13,774
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Schedule III
Deferred acquisition costs	258	246	240
Claims and claim adjustment expense reserves	3,421	3,323	3,615
Unearned premiums	1,515	1,444	1,437
Earned premiums	2,307	2,260	2,267
Net investment income	228	239	254
Claims and claim adjustment expenses	899	633	719
Amortization of deferred acquisition costs	432	421	418
Other operating expenses	464	445	440
Net written premiums	2,359	2,271	2,273
Reportable Segments [Member] | Personal Insurance [Member]
Schedule III
Deferred acquisition costs	707	651	601
Claims and claim adjustment expense reserves	5,860	5,051	4,674
Unearned premiums	4,543	4,160	3,793
Earned premiums	9,230	8,419	7,909
Net investment income	383	362	368
Claims and claim adjustment expenses	7,047	5,684	4,595
Amortization of deferred acquisition costs	1,448	1,343	1,285
Other operating expenses	1,111	1,124	1,118
Net written premiums	9,590	8,787	8,074
Other [Member]
Schedule III
Claims and claim adjustment expense reserves	17	20	23
Other operating expenses	$ 401	$ 394	$ 404