Business segment data - Insurance premiums by geographic area (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	$ 70,688	$ 65,275	$ 58,473
Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	5,780	6,294	6,428
United States [Member] | Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	59,648	55,451	50,250
United States [Member] | Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	2,120	2,161	2,820
Asia Pacific [Member] | Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	4,699	3,822	3,410
Asia Pacific [Member] | Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	1,704	2,030	1,652
Western Europe [Member] | Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	4,901	4,613	3,751
Western Europe [Member] | Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	1,235	1,298	1,120
All Other [Member] | Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	1,440	1,389	1,062
All Other [Member] | Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	$ 721	$ 805	$ 836