Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Net premiums written	$ 7,313	$ 7,104
Net premiums earned	7,137	7,027
Losses and loss expenses	4,098	4,102
Policy benefits	196	151
Policy acquisition costs	1,464	1,464
Administrative expenses	710	692
Underwriting income (loss)	669	618
Net investment income (loss)	836	806
Other (income) expense	(39)	(47)
Amortization of purchased intangibles	76	85
Segment Income (loss)	1,468	1,386
Net Realized Gains Losses	(97)	(2)
Interest expense	140	157
Chubb integration expenses	3	10
Income tax expense	188	135
Net income	1,040	1,082
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	2,951	2,812
Net premiums earned	3,085	3,029
Losses and loss expenses	1,973	1,908
Policy benefits	0	0
Policy acquisition costs	459	472
Administrative expenses	240	231
Underwriting income (loss)	413	418
Net investment income (loss)	510	503
Other (income) expense	(5)	(6)
Amortization of purchased intangibles	0	0
Segment Income (loss)	928	927
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,056	1,048
Net premiums earned	1,154	1,140
Losses and loss expenses	757	886
Policy benefits	0	0
Policy acquisition costs	231	237
Administrative expenses	68	65
Underwriting income (loss)	98	(48)
Net investment income (loss)	64	59
Other (income) expense	0	0
Amortization of purchased intangibles	3	3
Segment Income (loss)	159	8
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	130	108
Net premiums earned	55	43
Losses and loss expenses	(27)	(53)
Policy benefits	0	0
Policy acquisition costs	7	(1)
Administrative expenses	1	(3)
Underwriting income (loss)	74	100
Net investment income (loss)	10	7
Other (income) expense	0	0
Amortization of purchased intangibles	7	7
Segment Income (loss)	77	100
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	2,395	2,384
Net premiums earned	2,114	2,107
Losses and loss expenses	1,106	1,078
Policy benefits	0	0
Policy acquisition costs	596	588
Administrative expenses	249	239
Underwriting income (loss)	163	202
Net investment income (loss)	144	151
Other (income) expense	4	7
Amortization of purchased intangibles	11	10
Segment Income (loss)	292	336
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	202	193
Net premiums earned	168	168
Losses and loss expenses	76	67
Policy benefits	0	0
Policy acquisition costs	43	40
Administrative expenses	10	10
Underwriting income (loss)	39	51
Net investment income (loss)	56	64
Other (income) expense	(9)	(7)
Amortization of purchased intangibles	0	0
Segment Income (loss)	104	122
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	579	559
Net premiums earned	561	540
Losses and loss expenses	202	205
Policy benefits	196	151
Policy acquisition costs	128	128
Administrative expenses	79	78
Underwriting income (loss)	(44)	(22)
Net investment income (loss)	89	83
Other (income) expense	(40)	(4)
Amortization of purchased intangibles	0	1
Segment Income (loss)	85	64
Corporate Segment [Member]
Segment Reporting Information [Line Items]
Net premiums written	0	0
Net premiums earned	0	0
Losses and loss expenses	11	11
Policy benefits	0	0
Policy acquisition costs	0	0
Administrative expenses	63	72
Underwriting income (loss)	(74)	(83)
Net investment income (loss)	(37)	(61)
Other (income) expense	11	(37)
Amortization of purchased intangibles	55	64
Segment Income (loss)	(177)	(171)
Net Realized Gains Losses	(97)	(2)
Interest expense	140	157
Chubb integration expenses	3	10
Income tax expense	188	135
Net income	$ (605)	$ (475)