Investments (Detail) - Contractual maturities of available for sale fixed income securities (USD $) In Thousands	Jun. 30, 2011		Dec. 31, 2010
Due after 1 through 5 years	$ 8,381,553	[1]	$ 8,807,515	[1]
Due after 1 through 5 years	8,543,992	[1]	8,936,246	[1]
Due after 5 through 10 years	3,614,498	[1]	3,733,842	[1]
Due after 5 through 10 years	3,728,269	[1]	3,857,055	[1]
Due after 10 years	3,657,338	[1]	3,877,035	[1]
Due after 10 years	3,563,667	[1]	3,710,043	[1]
[AvailableForSaleSecuritiesDebtMaturitiesInFutureYearsAmortizedCost]	15,653,389	[1]	16,418,392	[1]
[AvailableForSaleSecuritiesDebtMaturitiesInFutureYearsValue]	15,835,928	[1]	16,503,344	[1]
Available for sale fixed income securities, Amortized Cost	25,105,488	[1]	25,714,886	[1]
Available for sale fixed income securities, Fair Value	25,151,686	[1]	25,544,179	[1]
Residential Mortgage Backed Securities Agency [Member]
Available for sale fixed income securities, Amortized Cost	5,506,311	[1]	5,020,469	[1]
Available for sale fixed income securities, Fair Value	5,676,340	[1]	5,164,746	[1]
Residential Mortgage Backed Securities Non Agency [Member]
Available for sale fixed income securities, Amortized Cost	970,742	[1]	1,256,741	[1]
Available for sale fixed income securities, Fair Value	773,286	[1]	1,021,088	[1]
Commercial Mortgage Backed Securities [Member]
Available for sale fixed income securities, Amortized Cost	1,097,687	[1]	1,135,075	[1]
Available for sale fixed income securities, Fair Value	1,146,402	[1]	1,172,507	[1]
Collateralized Debt Obligations [Member]
Available for sale fixed income securities, Amortized Cost	884,809	[1]	920,080	[1]
Available for sale fixed income securities, Fair Value	735,957	[1]	733,663	[1]
Other Asset Backed Securities [Member]
Available for sale fixed income securities, Amortized Cost	992,550	[1]	964,129	[1]
Available for sale fixed income securities, Fair Value	983,773	[1]	948,831	[1]
Mortgage And Asset Backed Securities [Member]
Available for sale fixed income securities, Amortized Cost	9,452,099	[1]	9,296,494	[1]
Available for sale fixed income securities, Fair Value	$ 9,315,758	[1]	$ 9,040,835	[1]

[1] Included in the table above are Tier One and Upper Tier Two securities, representing committed term debt and hybrid instruments senior to the common and preferred equities of the financial institutions, at their fair value of $641.9 million and $757.8 million at June 30, 2011 and December 31, 2010, respectively. These securities are reflected in the table based on their call date and have net unrealized losses of $104.2 million and $143.7 million at June 30, 2011 and December 31, 2010, respectively.