Investments (Detail) (USD $)	3 Months Ended			12 Months Ended											3 Months Ended							63 Months Ended
	Mar. 31, 2012		Mar. 31, 2011	Dec. 31, 2011		Mar. 31, 2012 Non-Agency RMBS [Member] Structured Credit Securities [Member]	Mar. 31, 2012 Core CDO Holdings Portfolio [Member] Structured Credit Securities [Member]	Mar. 31, 2012 CMBS Holdings Portfolio [Member] Structured Credit Securities [Member]	Mar. 31, 2012 Corporate Financial Instituion [Member] Life Operations Investment Portfolio [Member]	Mar. 31, 2012 Corporate Financial Instituion [Member] Non-Life Fixed Income Portfolios	Mar. 31, 2012 Hypothetical Interest and Credit Rate Increase Life Operations Investment Portfolio [Member]	Dec. 31, 2011 Hypothetical Interest and Credit Rate Increase Life Operations Investment Portfolio [Member]	Mar. 31, 2012 Corporate Financial Instituion [Member] Life Operations Investment Portfolio [Member]	Mar. 31, 2012 Tier One and Upper Tier Two Securities [Member] Life Operations Investment Portfolio [Member]	Mar. 31, 2012 Structured Credit Securities [Member]	Mar. 31, 2012 Corporate Securities, Including Medium Term Notes Backed Primarily By Investment Grade Eurpoean Credit [Member]	Mar. 31, 2012 Corporate Securities, Excluding Medium Term Notes Backed Primarily By Investment Grade European Credit [Member]	Mar. 31, 2012 Non-Agency RMBS [Member]	Mar. 31, 2011 Non-Agency RMBS [Member]	Mar. 31, 2012 Continual Unrealized Loss Position [Member]	Dec. 31, 2011 Continual Unrealized Loss Position [Member]	Mar. 31, 2012 Residential Mortgage-Backed Securities Non-Agency [Member]	Dec. 31, 2011 Residential Mortgage-Backed Securities Non-Agency [Member]	Mar. 31, 2012 Life Operations Investment Portfolio [Member]	Mar. 31, 2012 Topical Asset Portfolio [Member]	Mar. 31, 2012 Non-Life Fixed Income Portfolios Medium-term Notes [Member]	Mar. 31, 2012 Non-Life Fixed Income Portfolios	Mar. 31, 2012 Structured Credit Securities [Member]	Mar. 31, 2012 Hybrid Instrument [Member]	Dec. 31, 2011 Hybrid Instrument [Member]	Mar. 31, 2012 Government Related [Member]	Dec. 31, 2011 Government Related [Member]	Mar. 31, 2012 US Government Agencies Debt Securities [Member]	Dec. 31, 2011 US Government Agencies Debt Securities [Member]	Mar. 31, 2012 Covered Bonds Within Fixed Maturities [Member]	Dec. 31, 2011 Covered Bonds Within Fixed Maturities [Member]	Mar. 31, 2012 Covered Bonds Within Fixed Maturities And Short Term Investments [Member]	Dec. 31, 2011 Covered Bonds Within Fixed Maturities And Short Term Investments [Member]	Mar. 31, 2012 Medium-term Notes [Member]	Dec. 31, 2011 Medium-term Notes [Member]
Amortized Cost Of Investments Securities																															$ 2,272,100,000	$ 1,878,300,000	$ 461,200,000	$ 494,000,000
Investment In Debt Securities Fair Value																															2,313,400,000	1,915,600,000	499,400,000	541,200,000
Available-for-sale Debt Securities, Amortized Cost Basis				345,400,000																															398,600,000				220,000,000	297,700,000
Available-for-sale Securities, Fair Value Disclosure																																			415,500,000		33,900,000	44,700,000
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	2,769,800,000			2,668,978,000																															8,300,000	8,100,000
Held-to-maturity Securities, Fair Value	2,994,429,000			2,895,688,000																															7,700,000	7,700,000	7,700,000	7,700,000
Available-for-sale Securities, Debt Securities	27,291,662,000	[1]		26,190,025,000	[1],[2]																	632,054,000	641,815,000						411,200,000	386,100,000									198,700,000	266,000,000
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis	26,695,459,000	[1]		25,771,715,000	[1]																	792,800,000	851,557,000						489,300,000	494,900,000									220,000,000	297,700,000
Other Asset Backed Securities Covered Bonds Fair Value				353,900,000
Percentage of Fixed Income Investment portfolio invested in below investment grade or not rated	2.80%			2.40%
Percentage of Gross Unrealized losses in Fixed income securities portfolio related to securities below investment grade or not rated	33.60%			31.40%
Percentage Of Held To Maturity Securities Rated A Or Higher	97.80%
Investment Owned, Unrecognized Unrealized Appreciation	128,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position Transferred, Unamortized	110,500,000			108,400,000
Fixed Maturities, Tier One And Two Securities, Fair Value	411,200,000			386,100,000																411,200,000	386,100,000
Unrealized Gain (Loss) on Hybrid Instrument, Net	78,100,000			108,800,000
Pledged Assets Separately Reported, Other Debt Securities Available-for-sale or Held-for-investment	17,600,000,000			17,200,000,000
Medium Notes Fair Value																				198,700,000	266,000,000
Medium Notes Amortized Cost																				220,000,000	297,700,000
Fixed Maturities, Tier One and Two Securities, Amortized																				489,300,000	494,900,000
Fixed Maturities Equity Securities	98,000,000			91,600,000
Fixed Maturities Equity Securities Amortized Cost	100,000,000			100,000,000
Available-for-sale Securities, Gross Unrealized Losses	611,900,000
Available For Sale Securities Gross Unrealized Losses Considered As Temporarily Impaired Number Of Shares (in Shares)	1,543
Number Of Available For Sale Securities Held In Portfolio (in Shares)	7,349
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	9,200,000
Held To Maturity Securities Gross Unrealized Losses Considered As Temporarily Impaired Number Of Shares (in Shares)	23
Number Of Held To Maturity Securities Held In Portfolio (in Shares)	210
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value						183,100,000	160,000,000	5,100,000	160,700,000	45,700,000			89,200,000									184,100,000			160,000,000	22,100,000	96,600,000	65,500,000
Available-for-sale Securities	28,135,057,000			27,017,285,000					1,200,000,000					323,600,000								535,200,000		6,600,000,000	636,200,000	198,700,000	7,900,000,000	32,500,000
Available-for-sale Securities, Gross Realized Losses																						1,400,000,000
Held-to-maturity Securities	2,769,800,000			2,668,978,000							280,400,000	108,600,000
Percentage of Impairment on Amortized costs of Available for sale securities																												50.00%
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount															17,000,000	3,000,000	1,000,000
Impairments Related To Currency Losses	18,900,000		125,700,000
Credit Loss Impairments Previously Recognized On Securities Which Matured Paid Down Prepaid	$ (18,940,000)		$ (125,711,000)															$ 9,900,000	$ 91,700,000

[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.