Business Segment Data - Insurance premiums by geographic area (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting Information [Line Items]
Premiums written - Property/Casualty	$ 27,837	$ 25,834	$ 25,253
Premiums written - Life/Health	5,070	5,082	2,630
United States [Member]
Segment Reporting Information [Line Items]
Premiums written - Property/Casualty	22,253	21,539	19,280
Premiums written - Life/Health	3,100	3,210	1,095
Western Europe [Member]
Segment Reporting Information [Line Items]
Premiums written - Property/Casualty	4,495	3,377	5,236
Premiums written - Life/Health	880	945	761
All Other Geographic Areas [Member]
Segment Reporting Information [Line Items]
Premiums written - Property/Casualty	1,089	918	737
Premiums written - Life/Health	$ 1,090	$ 927	$ 774