Business segment data - Insurance premiums by geographic area (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	$ 84,074	$ 83,686	$ 80,339
Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	5,302	5,007	5,093
United States [Member] \| Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	72,461	71,723	67,831
United States [Member] \| Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	1,424	1,358	1,285
Western Europe [Member] \| Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	5,002	5,252	5,014
Western Europe [Member] \| Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	1,597	1,407	1,323
Asia Pacific [Member] \| Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	4,592	5,043	5,306
Asia Pacific [Member] \| Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	1,469	1,548	1,760
All Other [Member] \| Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	2,019	1,668	2,188
All Other [Member] \| Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	$ 812	$ 694	$ 725