Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Net premiums written	$ 9,199	$ 8,662
Net premiums earned	8,746	8,221
Losses and loss expenses	4,787	5,053
Policy benefits	145	167
Policy acquisition costs	1,737	1,665
Administrative expenses	778	744
Underwriting income (loss)	1,299	592
Net investment income (loss)	822	863
Other (income) expense	(310)	(490)
Amortization of purchased intangibles	71	72
Segment Income (loss)	2,360	1,873
Net realized gains (losses) (includes $(136) and $24 reclassified from AOCI)	101	887
Interest expense	132	122
Income tax expense	355	338
Net income (loss)	1,974	2,300
Segment Reconciling Items
Segment Reporting Information [Line Items]
Net premiums written	0	0
Net premiums earned	0	0
Losses and loss expenses	1	(1)
Policy benefits	(31)	4
Policy acquisition costs	0	0
Administrative expenses	0	0
Underwriting income (loss)	30	(3)
Net investment income (loss)	(63)	(41)
Other (income) expense	(32)	(45)
Amortization of purchased intangibles	0	0
Segment Income (loss)	(1)	1
Net realized gains (losses) (includes $(136) and $24 reclassified from AOCI)	1	(1)
Interest expense	0	0
Income tax expense	0	0
Net income (loss)	0	0
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	4,039	3,664
Net premiums earned	4,114	3,674
Losses and loss expenses	2,497	2,560
Policy benefits	0	0
Policy acquisition costs	573	514
Administrative expenses	265	254
Underwriting income (loss)	779	346
Net investment income (loss)	489	540
Other (income) expense	6	2
Amortization of purchased intangibles	0	0
Segment Income (loss)	1,262	884
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,180	1,098
Net premiums earned	1,247	1,184
Losses and loss expenses	713	819
Policy benefits	0	0
Policy acquisition costs	260	247
Administrative expenses	69	60
Underwriting income (loss)	205	58
Net investment income (loss)	59	65
Other (income) expense	1	1
Amortization of purchased intangibles	2	3
Segment Income (loss)	261	119
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	62	183
Net premiums earned	(29)	110
Losses and loss expenses	(92)	85
Policy benefits	0	0
Policy acquisition costs	12	12
Administrative expenses	(1)	3
Underwriting income (loss)	52	10
Net investment income (loss)	7	7
Other (income) expense	0	0
Amortization of purchased intangibles	7	7
Segment Income (loss)	52	10
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	3,079	2,890
Net premiums earned	2,628	2,478
Losses and loss expenses	1,389	1,263
Policy benefits	0	0
Policy acquisition costs	679	668
Administrative expenses	269	266
Underwriting income (loss)	291	281
Net investment income (loss)	147	141
Other (income) expense	2	1
Amortization of purchased intangibles	14	12
Segment Income (loss)	422	409
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	253	207
Net premiums earned	235	180
Losses and loss expenses	115	120
Policy benefits	0	0
Policy acquisition costs	62	45
Administrative expenses	9	8
Underwriting income (loss)	49	7
Net investment income (loss)	85	70
Other (income) expense	0	0
Amortization of purchased intangibles	0	0
Segment Income (loss)	134	77
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	586	620
Net premiums earned	551	595
Losses and loss expenses	154	198
Policy benefits	176	163
Policy acquisition costs	151	179
Administrative expenses	84	82
Underwriting income (loss)	(14)	(27)
Net investment income (loss)	103	98
Other (income) expense	(28)	(34)
Amortization of purchased intangibles	2	1
Segment Income (loss)	115	104
Segment Corporate and Other
Segment Reporting Information [Line Items]
Net premiums written	0	0
Net premiums earned	0	0
Losses and loss expenses	10	9
Policy benefits	0	0
Policy acquisition costs	0	0
Administrative expenses	83	71
Underwriting income (loss)	(93)	(80)
Net investment income (loss)	(5)	(17)
Other (income) expense	(259)	(415)
Amortization of purchased intangibles	46	49
Segment Income (loss)	115	269
Net realized gains (losses) (includes $(136) and $24 reclassified from AOCI)	100	888
Interest expense	132	122
Income tax expense	355	338
Net income (loss)	$ (272)	$ 697