Business segment data - Insurance premiums by type (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 - Direct	$ 65,495	$ 61,990	$ 56,700
Premiums written - Assumed	20,422	20,751	15,143
Premiums written - Ceded	(2,231)	(2,402)	(1,155)
Premiums written	83,686	80,339	70,688
Premiums earned - Direct	64,880	60,437	55,879
Premiums earned - Assumed	20,738	20,442	14,184
Premiums earned - Ceded	(2,359)	(2,548)	(1,293)
Premiums earned	83,259	78,331	68,770
Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written - Direct	19		582
Premiums written - Assumed	5,016	5,126	5,222
Premiums written - Ceded	(28)	(33)	(37)
Premiums written	5,007	5,093	5,767
Premiums earned - Direct	18		582
Premiums earned - Assumed	5,008	5,105	5,263
Premiums earned - Ceded	(28)	(33)	(39)
Premiums earned	$ 4,998	$ 5,072	$ 5,806