Segment Information (Net Premiums Earned For Segment By Product) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Net premiums earned	$ 49,846	$ 45,712	$ 40,360
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	20,008	18,416	17,107
North America Commercial P&C Insurance [Member] | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned	4,756	3,985	3,383
North America Commercial P&C Insurance [Member] | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	14,560	13,764	13,056
North America Commercial P&C Insurance [Member] | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned	692	667	668
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	6,188	5,536	5,180
North America Personal P&C Insurance [Member] | Personal automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned	968	859	811
North America Personal P&C Insurance [Member] | Personal homeowners [Member]
Segment Reporting Information [Line Items]
Net premiums earned	4,293	3,833	3,557
North America Personal P&C Insurance [Member] | Insurance, Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	927	844	812
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	2,705	3,169	2,838
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	13,400	12,231	10,803
Overseas General Insurance [Member] | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned	4,338	3,831	3,382
Overseas General Insurance [Member] | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	3,705	3,526	3,232
Overseas General Insurance [Member] | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned	2,572	2,469	2,169
Overseas General Insurance [Member] | Personal lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	2,785	2,405	2,020
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	1,272	962	922
Global Reinsurance [Member] | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned	490	331	211
Global Reinsurance [Member] | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	550	472	503
Global Reinsurance [Member] | Property catastrophe [Member]
Segment Reporting Information [Line Items]
Net premiums earned	232	159	208
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	6,273	5,398	3,510
Life Insurance [Member] | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned	3,224	3,097	2,055
Life Insurance [Member] | Life [Member]
Segment Reporting Information [Line Items]
Net premiums earned	$ 3,049	$ 2,301	$ 1,455