Business segment data - Insurance premiums by type (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 - Direct	$ 56,700	$ 53,829	$ 47,838
Premiums written - Assumed	15,143	12,461	11,533
Premiums written - Ceded	(1,155)	(1,015)	(898)
Premiums written	70,688	65,275	58,473
Premiums earned - Direct	55,879	52,139	46,418
Premiums earned - Assumed	14,184	12,072	11,449
Premiums earned - Ceded	(1,293)	(1,054)	(907)
Premiums earned	68,770	63,157	56,960
Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written - Direct	582	649	510
Premiums written - Assumed	5,235	5,685	5,960
Premiums written - Ceded	(37)	(40)	(42)
Premiums written	5,780	6,294	6,428
Premiums earned - Direct	582	649	510
Premiums earned - Assumed	5,332	5,713	5,973
Premiums earned - Ceded	(39)	(41)	(42)
Premiums earned	$ 5,875	$ 6,321	$ 6,441