Business segment data - Insurance premiums by geographic area (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	$ 65,275	$ 58,473	$ 56,971
Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	6,294	6,428	5,840
United States [Member] | Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	55,451	50,250	50,529
United States [Member] | Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	2,161	2,820	2,553
Asia Pacific [Member] | Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	3,822	3,410	3,114
Asia Pacific [Member] | Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	2,030	1,652	1,582
Western Europe [Member] | Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	4,613	3,751	2,535
Western Europe [Member] | Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	1,298	1,120	908
All Other [Member] | Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	1,389	1,062	793
All Other [Member] | Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	$ 805	$ 836	$ 797