Business segment data - Insurance premiums by type (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written - Direct	$ 44,513	$ 39,377	$ 34,001
Premiums written - Assumed	8,970	17,815	8,037
Premiums written - Ceded	(869)	(694)	(798)
Premiums written	52,614	56,498	41,240
Premiums earned - Direct	43,095	37,755	33,207
Premiums earned - Assumed	8,649	17,813	7,848
Premiums earned - Ceded	(825)	(677)	(843)
Premiums earned	50,919	54,891	40,212
Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written - Direct	1,111	866	1,060
Premiums written - Assumed	5,540	4,925	4,672
Premiums written - Ceded	(49)	(47)	(62)
Premiums written	6,602	5,744	5,670
Premiums earned - Direct	1,111	866	1,060
Premiums earned - Assumed	5,438	4,866	4,671
Premiums earned - Ceded	(50)	(26)	(62)
Premiums earned	$ 6,499	$ 5,706	$ 5,669