Business segment data - Insurance premiums by type (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 - Direct	$ 61,990	$ 56,700	$ 53,829
Premiums written - Assumed	20,751	15,143	12,461
Premiums written - Ceded	(2,402)	(1,155)	(1,015)
Premiums written	80,339	70,688	65,275
Premiums earned - Direct	60,437	55,879	52,139
Premiums earned - Assumed	20,442	14,184	12,072
Premiums earned - Ceded	(2,548)	(1,293)	(1,054)
Premiums earned	78,331	68,770	63,157
Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written - Direct		582	646
Premiums written - Assumed	5,126	5,222	5,670
Premiums written - Ceded	(33)	(37)	(40)
Premiums written	5,093	5,767	6,276
Premiums earned - Direct		582	646
Premiums earned - Assumed	5,105	5,263	5,698
Premiums earned - Ceded	(33)	(39)	(41)
Premiums earned	$ 5,072	$ 5,806	$ 6,303