Business segment data - Insurance premiums by geographic area (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	$ 80,339	$ 70,688	$ 65,275
Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	5,093	5,767	6,276
United States [Member] | Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	67,831	59,648	55,451
United States [Member] | Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	1,285	2,107	2,143
Asia Pacific [Member] | Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	5,306	4,699	3,822
Asia Pacific [Member] | Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	1,760	1,704	2,030
Western Europe [Member] | Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	5,014	4,901	4,613
Western Europe [Member] | Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	1,323	1,235	1,298
All Other [Member] | Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	2,188	1,440	1,389
All Other [Member] | Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	$ 725	$ 721	$ 805