Business segment data - Insurance premiums by geographic area (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Premiums written - Property/Casualty	$ 29,892	$ 27,837	$ 25,834
Premiums written - Life/Health	5,835	5,070	5,082
United States [Member]
Segment Reporting Information [Line Items]
Premiums written - Property/Casualty	23,186	22,253	21,539
Premiums written - Life/Health	3,504	3,100	3,210
Western Europe [Member]
Segment Reporting Information [Line Items]
Premiums written - Property/Casualty	4,387	4,495	3,377
Premiums written - Life/Health	1,114	880	945
All Other Geographic Areas [Member]
Segment Reporting Information [Line Items]
Premiums written - Property/Casualty	2,319	1,089	918
Premiums written - Life/Health	$ 1,217	$ 1,090	$ 927