Assumed Life Reinsurance Programs Involving Minimum Benefit Guarantees Under Annuity Contracts (Narrative) (Detail) (USD $)	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012 Guaranteed Minimum Death Benefit [Member]	Dec. 31, 2011 Guaranteed Minimum Death Benefit [Member]	Sep. 30, 2012 Guaranteed Minimum Income Benefit [Member]	Dec. 31, 2011 Guaranteed Minimum Income Benefit [Member]	Sep. 30, 2011 Guaranteed Minimum Income Benefit [Member]	Jun. 30, 2011 Guaranteed Minimum Income Benefit [Member]	Dec. 31, 2010 Guaranteed Minimum Income Benefit [Member]	Sep. 30, 2012 Guaranteed Minimum Deaths Benefits And Guaranteed Living Benefits [Member]	Dec. 31, 2011 Guaranteed Minimum Deaths Benefits And Guaranteed Living Benefits [Member]	Sep. 30, 2012 Guaranteed Living Benefits [Member]	Dec. 31, 2011 Guaranteed Living Benefits [Member]
Guaranteed Minimum Benefits [Line Items]
Reported liabilities		$ 114,000,000	$ 138,000,000	$ 1,500,000,000	$ 1,500,000,000	$ 1,635,000,000	$ 676,000,000	$ 648,000,000
Fair value derivative adjustment in liability				1,300,000,000
Net amount at risk		1,400,000,000	1,800,000,000								382,000,000	380,000,000
Mortality percentage according to Annuity 2000 mortality table		100.00%
Discounting assumption used in the calculation of the benefit reserve averaging - lower range		1.00%									3.50%
Discounting assumption used in the calculation of the benefit reserve averaging - upper range		2.00%									4.50%
Total claim amount payable, if all of the Company's cedants' policyholders covered were to die immediately		486,000,000							885,000,000
GMBD net amount of risk									137,000,000	182,000,000
GLB net amount of risk									$ 825,000,000	$ 998,000,000
Average attained age of all policyholders under all benefits reinsured, years	68 years