Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Net premiums written	$ 7,977	$ 7,313
Net premiums earned	7,794	7,137
Losses and loss expenses	4,485	4,098
Policy benefits	129	196
Policy acquisition costs	1,615	1,464
Administrative expenses	741	710
Underwriting income (loss)	824	669
Net investment income (loss)	861	836
Other (income) expense	55	(39)
Amortization of purchased intangibles	73	76
Segment Income (loss)	1,557	1,468
Net Realized Gains Losses	(958)	(97)
Interest expense	132	140
Chubb integration expenses	0	3
Income tax expense	215	188
Net income	252	1,040
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	3,252	2,951
Net premiums earned	3,376	3,085
Losses and loss expenses	2,181	1,973
Policy benefits	0	0
Policy acquisition costs	492	459
Administrative expenses	259	240
Underwriting income (loss)	444	413
Net investment income (loss)	516	510
Other (income) expense	(3)	(5)
Amortization of purchased intangibles	0	0
Segment Income (loss)	963	928
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,107	1,056
Net premiums earned	1,200	1,154
Losses and loss expenses	683	757
Policy benefits	0	0
Policy acquisition costs	245	231
Administrative expenses	68	68
Underwriting income (loss)	204	98
Net investment income (loss)	66	64
Other (income) expense	2	0
Amortization of purchased intangibles	3	3
Segment Income (loss)	265	159
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	157	130
Net premiums earned	94	55
Losses and loss expenses	63	(27)
Policy benefits	0	0
Policy acquisition costs	11	7
Administrative expenses	4	1
Underwriting income (loss)	16	74
Net investment income (loss)	9	10
Other (income) expense	0	0
Amortization of purchased intangibles	7	7
Segment Income (loss)	18	77
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	2,598	2,395
Net premiums earned	2,307	2,114
Losses and loss expenses	1,258	1,106
Policy benefits	0	0
Policy acquisition costs	642	596
Administrative expenses	258	249
Underwriting income (loss)	149	163
Net investment income (loss)	145	144
Other (income) expense	4	4
Amortization of purchased intangibles	12	11
Segment Income (loss)	278	292
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	218	202
Net premiums earned	186	168
Losses and loss expenses	87	76
Policy benefits	0	0
Policy acquisition costs	45	43
Administrative expenses	10	10
Underwriting income (loss)	44	39
Net investment income (loss)	54	56
Other (income) expense	(15)	(9)
Amortization of purchased intangibles	0	0
Segment Income (loss)	113	104
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	645	579
Net premiums earned	631	561
Losses and loss expenses	202	202
Policy benefits	129	196
Policy acquisition costs	180	128
Administrative expenses	76	79
Underwriting income (loss)	44	(44)
Net investment income (loss)	95	89
Other (income) expense	44	(40)
Amortization of purchased intangibles	1	0
Segment Income (loss)	94	85
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	66	63
Underwriting income (loss)	(77)	(74)
Net investment income (loss)	(24)	(37)
Other (income) expense	23	11
Amortization of purchased intangibles	50	55
Segment Income (loss)	(174)	(177)
Net Realized Gains Losses	(958)	(97)
Interest expense	132	140
Chubb integration expenses		3
Income tax expense	215	188
Net income	$ (1,479)	$ (605)