Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Net premiums written	$ 51,468	$ 47,361	$ 41,720
Net premiums earned	49,846	45,712	40,360
Losses and loss expenses	26,022	24,100	22,572
Policy acquisition costs	9,102	8,259	7,339
Administrative expenses	4,380	4,007	3,395
Other (income) expense	(1,023)	(836)	89
Amortization of purchased intangibles	323	310	285
Segment income	12,975	11,630	9,319
Net realized gains (losses)	117	(607)	(1,085)
Market risk benefits gains (losses)	(140)	(307)	80
Interest expense	741	672	570
Integration expenses	39	69	48
Other reclassification	(208)	5	11
Income before income tax	11,455	9,526	6,485
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	20,589	19,237	17,889
Net premiums earned	20,008	18,416	17,107
Losses and loss expenses	12,737	11,256	10,828
Policy benefits	0	0	0
Policy acquisition costs	2,718	2,515	2,313
Administrative expenses	1,337	1,250	1,113
Underwriting Income (Loss)	3,216	3,395	2,853
Net investment income	3,556	3,017	2,247
Other (income) expense	32	22	17
Amortization of purchased intangibles	3	0	0
Segment income	6,737	6,390	5,083
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	6,532	5,878	5,313
Net premiums earned	6,188	5,536	5,180
Losses and loss expenses	3,584	3,511	3,186
Policy benefits	0	0	0
Policy acquisition costs	1,239	1,128	1,057
Administrative expenses	351	329	291
Underwriting Income (Loss)	1,014	568	646
Net investment income	433	358	283
Other (income) expense	1	3	4
Amortization of purchased intangibles	9	9	10
Segment income	1,437	914	915
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	2,703	3,188	2,907
Net premiums earned	2,705	3,169	2,838
Losses and loss expenses	2,170	2,874	2,557
Policy benefits	0	0	0
Policy acquisition costs	191	150	126
Administrative expenses	(10)	(1)	(10)
Underwriting Income (Loss)	354	146	165
Net investment income	84	63	36
Other (income) expense	1	1	1
Amortization of purchased intangibles	25	25	26
Segment income	412	183	174
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	13,972	12,575	11,060
Net premiums earned	13,400	12,231	10,803
Losses and loss expenses	6,414	5,643	4,894
Policy benefits	408	457	358
Policy acquisition costs	3,410	3,113	2,818
Administrative expenses	1,351	1,219	1,070
Underwriting Income (Loss)	1,817	1,799	1,663
Net investment income	1,136	895	626
Other (income) expense	14	(25)	2
Amortization of purchased intangibles	81	70	57
Segment income	2,858	2,649	2,230
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,346	1,018	943
Net premiums earned	1,272	962	922
Losses and loss expenses	711	426	670
Policy benefits	0	0	0
Policy acquisition costs	342	264	240
Administrative expenses	39	37	36
Underwriting Income (Loss)	180	235	(24)
Net investment income	253	208	281
Other (income) expense	0	(2)	1
Amortization of purchased intangibles	0	0	0
Segment income	433	445	256
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	6,326	5,465	3,608
Net premiums earned	6,273	5,398	3,510
Losses and loss expenses	112	114	85
Policy benefits	4,101	3,216	1,998
Policy acquisition costs	1,202	1,089	785
Administrative expenses	880	771	510
Net investment income	1,003	756	509
Other (income) expense	(159)	(115)	(30)
Amortization of purchased intangibles	42	30	10
Segment income	1,098	1,049	661
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Underwriting Income (Loss)	(731)	(683)	(748)
Net investment income	(105)	25
Other (income) expense	(490)	(380)	292
Amortization of purchased intangibles	$ 163	$ 176	$ 182