Business segment data - Insurance premiums by type (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written - Direct	$ 34,001	$ 30,544	$ 27,541
Premiums written - Assumed	8,037	7,049	9,889
Premiums written - Ceded	(798)	(877)	(839)
Premiums written	41,240	36,716	36,591
Premiums earned - Direct	33,207	29,608	26,389
Premiums earned - Assumed	7,848	6,584	9,872
Premiums earned - Ceded	(843)	(854)	(850)
Premiums earned	40,212	35,338	35,411
Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written - Direct	1,060	821	879
Premiums written - Assumed	4,672	5,187	5,030
Premiums written - Ceded	(62)	(57)	(67)
Premiums written	5,670	5,951	5,842
Premiums earned - Direct	1,060	821	879
Premiums earned - Assumed	4,671	5,192	5,030
Premiums earned - Ceded	(62)	(57)	(67)
Premiums earned	$ 5,669	$ 5,956	$ 5,842