Fair Value Measurements (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2012		Dec. 31, 2011
Available-for-sale Securities, Debt Securities	$ 27,291,662,000	[1]	$ 26,190,025,000	[1],[2]
Available-for-sale Debt Securities, Amortized Cost Basis			345,400,000
Total Fixed Maturities Fair Value Included Within Other Asset Backed Securities	415,500,000
Total Fixed Maturities Fair Value Reclassified From Corporate To Other Asset Backed Securities			353,900,000
Available-for-sale Securities, Equity Securities	631,068,000	[3]	468,197,000	[3]
Investments In Structured Transactions not Carried at Fair Value	316,900,000		323,700,000
Discount rate for determining fair value of deposit liabilities	102.5		161.8
Medium-term Notes [Member]
Available-for-sale Securities, Debt Securities	198,700,000		266,000,000
Available-for-sale Debt Securities, Amortized Cost Basis	220,000,000		297,700,000
Fixed Income Securities [Member]
Available-for-sale Securities, Equity Securities	98,000,000		91,600,000
Netting [Member]
Other Assets And Liabilities Net Fair Value Disclosure	$ 77,500,000		$ 77,100,000

[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.

[3] Included within equity securities are investments in fixed income funds of $98.0 million and $91.6 million at March 31, 2012 and December 31, 2011, respectively.