Business Segment Data - Insurance premiums by type (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 - Direct - Life/Health	$ 67	$ 3
Premiums written - Assumed - Life/Health	5,133	5,203	2,727
Premiums written - Ceded - Life/Health	(130)	(124)	(97)
Premiums written - Life/Health	5,070	5,082	2,630
Premiums earned - Direct - Life/Health	67	3
Premiums earned - Assumed - Life/Health	5,099	5,208	2,723
Premiums earned - Ceded - Life/Health	(130)	(124)	(97)
Premiums earned - Life/Health	5,036	5,087	2,626
Premiums written - Direct - Property/Casualty	18,512	17,128	16,484
Premiums written - Assumed - Property/Casualty	9,867	9,171	9,321
Premiums written - Ceded - Property/Casualty	(542)	(465)	(552)
Premiums written - Property/Casualty	27,837	25,834	25,253
Premiums earned - Direct - Property/Casualty	18,038	16,932	16,553
Premiums earned - Assumed - Property/Casualty	9,523	9,266	9,284
Premiums earned - Ceded - Property/Casualty	(522)	(536)	(579)
Premiums earned - Property/Casualty	$ 27,039	$ 25,662	$ 25,258