Investments (Detail) - Contractual maturities of available for sale securities and held to maturity income securities (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012		Dec. 31, 2011
Fixed maturities - AFS
Due less than one year	$ 1,888,009	[1]	$ 2,004,395	[1]
Due less than one year	1,902,217	[1]	2,020,361	[1]
Due after 1 through 5 years	8,510,647	[1]	7,736,717	[1]
Due after 1 through 5 years	8,768,164	[1]	7,909,354	[1]
Due after 5 through 10 years	3,745,093	[1]	3,619,141	[1]
Due after 5 through 10 years	3,930,750	[1]	3,777,073	[1]
Due after 10 years	3,416,477	[1]	3,257,886	[1]
Due after 10 years	3,648,226	[1]	3,488,330	[1]
	17,560,226	[1]	16,618,139	[1]
	18,249,357	[1]	17,195,118	[1]
Available for sale fixed income securities, Amortized Cost	26,695,459	[1]	25,771,715	[1]
Available for sale fixed income securities, Fair Value	27,291,662	[1]	26,190,025	[1],[2]
Fixed maturities - HTM
Due less than one year	35,696	[1]	11,796	[1]
Due less than one year	35,831	[1]	11,768	[1]
Due after 1 through 5 years	134,543	[1]	122,091	[1]
Due after 1 through 5 years	138,274	[1]	123,871	[1]
Due after 5 through 10 years	398,290	[1]	393,865	[1]
Due after 5 through 10 years	418,132	[1]	402,424	[1]
Due after 10 years	1,883,179	[1]	1,771,530	[1]
Due after 10 years	2,065,346	[1]	1,960,886	[1]
	2,451,708	[1]	2,299,282	[1]
	2,657,583	[1]	2,498,949	[1]
Held to maturity income securities, Amortized Cost	2,769,800	[1]	2,668,978	[1]
Held to maturity income securities, Fair Value	2,994,429	[1]	2,895,688	[1]
Residential Mortgage Backed Securities [Member] | Agency Securities [Member]
Fixed maturities - AFS
Available for sale fixed income securities, Amortized Cost	5,238,075	[1]	5,189,473	[1]
Available for sale fixed income securities, Fair Value	5,393,391	[1]	5,379,406	[1]
Residential Mortgage Backed Securities [Member] | Non-Agency [Member]
Fixed maturities - AFS
Available for sale fixed income securities, Amortized Cost	792,800	[1]	851,557	[1]
Available for sale fixed income securities, Fair Value	632,054	[1]	641,815	[1]
Fixed maturities - HTM
Held to maturity income securities, Amortized Cost	83,023	[1]	80,955	[1]
Held to maturity income securities, Fair Value	87,135	[1]	87,443	[1]
Commercial Mortgage Backed Securities [Member]
Fixed maturities - AFS
Available for sale fixed income securities, Amortized Cost	882,762	[1]	927,684	[1]
Available for sale fixed income securities, Fair Value	944,153	[1]	974,835	[1]
Fixed maturities - HTM
Held to maturity income securities, Amortized Cost	12,912	[1]	0	[1]
Held to maturity income securities, Fair Value	14,123	[1]	0	[1]
Collateralized Debt Obligations [Member]
Fixed maturities - AFS
Available for sale fixed income securities, Amortized Cost	798,313	[1]	843,553	[1]
Available for sale fixed income securities, Fair Value	646,565	[1]	658,602	[1]
Other Asset Backed Securities [Member]
Fixed maturities - AFS
Available for sale fixed income securities, Amortized Cost	1,423,283	[1]	1,341,309	[1]
Available for sale fixed income securities, Fair Value	1,426,142	[1]	1,340,249	[1]
Fixed maturities - HTM
Held to maturity income securities, Amortized Cost	222,157	[1]	288,741	[1]
Held to maturity income securities, Fair Value	235,588	[1]	309,296	[1]
Mortgage and Asset Backed Securities [Member]
Fixed maturities - AFS
Available for sale fixed income securities, Amortized Cost	9,135,233	[1]	9,153,576	[1]
Available for sale fixed income securities, Fair Value	9,042,305	[1]	8,994,907	[1]
Fixed maturities - HTM
Held to maturity income securities, Amortized Cost	318,092	[1]	369,696	[1]
Held to maturity income securities, Fair Value	$ 336,846	[1]	$ 396,739	[1]

[1] Included in the table above are Tier One and Upper Tier Two securities, representing committed term debt and hybrid instruments, which are senior to the common and preferred equities of the financial institutions, at their fair value of $411.2 million and $386.1 million at March 31, 2012 and December 31, 2011, respectively. These securities are reflected in the table based on their call date and have net unrealized losses of $78.1 million and $108.8 million at March 31, 2012 and December 31, 2011, respectively.

[2] The Company invests in covered bonds issued by financial institutions ("Covered Bonds"). Covered Bonds are senior secured debt instruments issued by financial institutions and backed by over-collateralized pools of public sector or mortgage loans. During the three months ended March 31, 2012, Covered Bonds within Total fixed maturities with a fair value of $415.5 million, have been included within Other asset-backed securities to align the Company's classification to market indices. At December 31, 2011 Covered Bonds within Total fixed maturities with a fair value of $353.9 million, have been reclassified from Corporate to Other asset-backed securities to conform to current period presentation.