Business segment data - Insurance premiums by geographic area (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	$ 41,240	$ 36,716	$ 36,591
Property/Casualty [Member] | United States [Member]
Segment Reporting Information [Line Items]
Premiums written	35,878	31,171	31,362
Property/Casualty [Member] | Asia Pacific [Member]
Segment Reporting Information [Line Items]
Premiums written	3,616	3,472	1,953
Property/Casualty [Member] | Western Europe [Member]
Segment Reporting Information [Line Items]
Premiums written	1,406	1,638	2,424
Property/Casualty [Member] | All Other Geographic Areas [Member]
Segment Reporting Information [Line Items]
Premiums written	340	435	852
Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	5,670	5,951	5,842
Life/Health [Member] | United States [Member]
Segment Reporting Information [Line Items]
Premiums written	3,473	3,247	3,402
Life/Health [Member] | Asia Pacific [Member]
Segment Reporting Information [Line Items]
Premiums written	715	673	651
Life/Health [Member] | Western Europe [Member]
Segment Reporting Information [Line Items]
Premiums written	822	1,263	1,135
Life/Health [Member] | All Other Geographic Areas [Member]
Segment Reporting Information [Line Items]
Premiums written	$ 660	$ 768	$ 654