Business segment data - Insurance premiums by geographic area (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	$ 36,716	$ 36,591	$ 30,911
Property/Casualty [Member] | United States [Member]
Segment Reporting Information [Line Items]
Premiums written	31,171	31,362	25,704
Property/Casualty [Member] | Asia Pacific [Member]
Segment Reporting Information [Line Items]
Premiums written	3,472	1,953	2,353
Property/Casualty [Member] | Western Europe [Member]
Segment Reporting Information [Line Items]
Premiums written	1,638	2,424	2,234
Property/Casualty [Member] | All Other Geographic Areas [Member]
Segment Reporting Information [Line Items]
Premiums written	435	852	620
Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	5,951	5,842	6,299
Life/Health [Member] | United States [Member]
Segment Reporting Information [Line Items]
Premiums written	3,247	3,402	3,934
Life/Health [Member] | Asia Pacific [Member]
Segment Reporting Information [Line Items]
Premiums written	673	651	420
Life/Health [Member] | Western Europe [Member]
Segment Reporting Information [Line Items]
Premiums written	1,263	1,135	1,339
Life/Health [Member] | All Other Geographic Areas [Member]
Segment Reporting Information [Line Items]
Premiums written	$ 768	$ 654	$ 606