Business segment data - Insurance premiums by type (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written - Direct	$ 47,578	$ 44,513	$ 39,377
Premiums written - Assumed	10,214	8,970	17,815
Premiums written - Ceded	(821)	(869)	(694)
Premiums written	56,971	52,614	56,498
Premiums earned - Direct	46,540	43,095	37,755
Premiums earned - Assumed	9,643	8,649	17,813
Premiums earned - Ceded	(851)	(825)	(677)
Premiums earned	55,332	50,919	54,891
Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written - Direct	839	1,111	866
Premiums written - Assumed	5,046	5,540	4,925
Premiums written - Ceded	(45)	(49)	(47)
Premiums written	5,840	6,602	5,744
Premiums earned - Direct	839	1,111	866
Premiums earned - Assumed	4,952	5,438	4,866
Premiums earned - Ceded	(45)	(50)	(26)
Premiums earned	$ 5,746	$ 6,499	$ 5,706