Business segment data - Insurance premiums by type (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written - Direct	$ 39,377	$ 34,001	$ 30,544
Premiums written - Assumed	17,815	8,037	7,049
Premiums written - Ceded	(694)	(798)	(877)
Premiums written	56,498	41,240	36,716
Premiums earned - Direct	37,755	33,207	29,608
Premiums earned - Assumed	17,813	7,848	6,584
Premiums earned - Ceded	(677)	(843)	(854)
Premiums earned	54,891	40,212	35,338
Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written - Direct	866	1,060	821
Premiums written - Assumed	4,925	4,672	5,187
Premiums written - Ceded	(47)	(62)	(57)
Premiums written	5,744	5,670	5,951
Premiums earned - Direct	866	1,060	821
Premiums earned - Assumed	4,866	4,671	5,192
Premiums earned - Ceded	(26)	(62)	(57)
Premiums earned	$ 5,706	$ 5,669	$ 5,956