Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Net premiums written	$ 6,710	$ 5,995
Net premiums earned	6,772	6,597
Losses and loss expenses	3,789	3,674
Policy benefits	168	126
Policy acquisition costs	1,397	1,413
Administrative expenses	676	772
Underwriting income (loss)	742	612
Net investment income	745	674
Other (income) expense	(70)	28
Amortization of purchased intangibles	64	7
Segment Income (loss)	1,493	1,251
Net realized gains (losses) including OTTI	(7)	(394)
Interest expense	154	146
Chubb integration expenses	111	148
Income tax expense	128	124
Net income	1,093	439
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	2,742	2,302
Net premiums earned	3,041	2,896
Losses and loss expenses	1,860	1,747
Policy benefits	0	0
Policy acquisition costs	487	482
Administrative expenses	231	266
Underwriting income (loss)	463	401
Net investment income	478	426
Other (income) expense	4	0
Amortization of purchased intangibles	0	0
Segment Income (loss)	937	827
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	984	871
Net premiums earned	1,086	1,024
Losses and loss expenses	633	661
Policy benefits	0	0
Policy acquisition costs	217	249
Administrative expenses	65	88
Underwriting income (loss)	171	26
Net investment income	55	47
Other (income) expense	1	1
Amortization of purchased intangibles	3	8
Segment Income (loss)	222	64
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	61	64
Net premiums earned	14	23
Losses and loss expenses	(73)	(30)
Policy benefits	0	0
Policy acquisition costs	(1)	4
Administrative expenses	(5)	(4)
Underwriting income (loss)	93	53
Net investment income	6	5
Other (income) expense	0	0
Amortization of purchased intangibles	7	7
Segment Income (loss)	92	51
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	2,200	2,041
Net premiums earned	1,936	1,955
Losses and loss expenses	1,071	1,021
Policy benefits	0	0
Policy acquisition costs	529	503
Administrative expenses	245	263
Underwriting income (loss)	91	168
Net investment income	148	146
Other (income) expense	(1)	(5)
Amortization of purchased intangibles	11	11
Segment Income (loss)	229	308
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	199	201
Net premiums earned	189	202
Losses and loss expenses	94	89
Policy benefits	0	0
Policy acquisition costs	51	53
Administrative expenses	10	14
Underwriting income (loss)	34	46
Net investment income	62	67
Other (income) expense	0	(1)
Amortization of purchased intangibles	0	0
Segment Income (loss)	96	114
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	524	516
Net premiums earned	506	497
Losses and loss expenses	193	177
Policy benefits	168	126
Policy acquisition costs	114	122
Administrative expenses	72	72
Underwriting income (loss)	(41)	0
Net investment income	75	67
Other (income) expense	(29)	6
Amortization of purchased intangibles	1	1
Segment Income (loss)	62	60
Corporate And Other [Member]
Segment Reporting Information [Line Items]
Net premiums written	0	0
Net premiums earned	0	0
Losses and loss expenses	11	9
Policy benefits	0	0
Policy acquisition costs	0	0
Administrative expenses	58	73
Underwriting income (loss)	(69)	(82)
Net investment income	(79)	(84)
Other (income) expense	(45)	27
Amortization of purchased intangibles	42	(20)
Segment Income (loss)	(145)	(173)
Net realized gains (losses) including OTTI	(7)	(394)
Interest expense	154	146
Chubb integration expenses	111	148
Income tax expense	128	124
Net income	$ (545)	$ (985)