Business segment data - Insurance premiums by geographic area (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	$ 83,686	$ 80,339	$ 70,688
Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	5,007	5,093	5,767
United States [Member] | Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	71,723	67,831	59,648
United States [Member] | Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	1,358	1,285	2,107
Western Europe [Member] | Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	5,252	5,014	4,901
Western Europe [Member] | Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	1,407	1,323	1,235
Asia Pacific [Member] | Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	5,043	5,306	4,699
Asia Pacific [Member] | Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	1,548	1,760	1,704
All Other [Member] | Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	1,668	2,188	1,440
All Other [Member] | Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	$ 694	$ 725	$ 721