Investments (Detail) (USD $)	3 Months Ended			9 Months Ended			12 Months Ended											3 Months Ended	9 Months Ended		3 Months Ended								57 Months Ended
	Sep. 30, 2011		Sep. 30, 2010	Sep. 30, 2011		Sep. 30, 2010	Dec. 31, 2010		Nov. 01, 2009	Sep. 30, 2011 Medium-term Notes [Member] Non-Life Fixed Income Portfolios	Dec. 31, 2010 Continual Unrealized Loss Position [Member]	Sep. 30, 2011 Continual Unrealized Loss Position [Member]	Dec. 31, 2010 Continual Unrealized Loss Position [Member]	Sep. 30, 2011 Corporate Financial Instituion [Member] Life Operations Investment Portfolio [Member]	Sep. 30, 2011 Corporate Financial Instituion [Member] Non-Life Fixed Income Portfolios	Sep. 30, 2011 Tier One and Upper Tier Two Securities [Member] Life Operations Investment Portfolio [Member]	Sep. 30, 2011 Hypothetical Interest And Credit Rate Increase [Member] Life Operations Investment Portfolio [Member]	Sep. 30, 2011 Non-Agency RMBS [Member]	Sep. 30, 2011 Non-Agency RMBS [Member]	Sep. 30, 2011 Non-Agency RMBS [Member] Structured Credit Securities [Member]	Sep. 30, 2011 Corporate Securities Excluding MediumTerm Notes Backed Primarily By Investment Grade Eurpoean Credit [Member]	Sep. 30, 2011 Corporate Securities, Including Medium Term Notes Backed Primarily By Investment Grade Eurpoean Credit [Member]	Sep. 30, 2011 CMBS Holdings Portfolio [Member] Structured Credit Securities [Member]	Sep. 30, 2011 Core CDO Holdings Portfolio [Member] Structured Credit Securities [Member]	Sep. 30, 2011 Government Related [Member]	Dec. 31, 2010 Government Related [Member]	Sep. 30, 2011 US Government Agencies Debt Securities [Member]	Dec. 31, 2010 US Government Agencies Debt Securities [Member]	Sep. 30, 2011 Residential Mortgage-Backed Securities Non-Agency [Member]	Sep. 30, 2011 Life Operations Investment Portfolio [Member]	Sep. 30, 2011 Topical Asset Portfolio [Member]	Sep. 30, 2011 Non-Life Fixed Income Portfolios	Sep. 30, 2011 Structured Credit Securities [Member]
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis	$ 26,750,242,000	[1]		$ 26,750,242,000	[1]		$ 27,322,842,000	[1]																	$ 2,022,700,000	$ 2,101,000,000	$ 531,000,000	$ 1,019,200,000
Available-for-sale Securities, Debt Securities	27,005,350,000	[1]		27,005,350,000	[1]		27,142,105,000	[1]																	2,058,500,000	2,131,200,000	581,500,000	1,072,600,000
Medium Notes Fair Value	429,200,000			429,200,000																									429,200,000
Medium Notes Amortized Cost	460,000,000			460,000,000								460,000,000
Fixed Maturities, Tier One And Two Securities, Fair Value	485,600,000			485,600,000							757,800,000	653,200,000
Fixed Maturities, Tier One and Two Securities, Amortized	653,200,000			653,200,000							883,000,000
Fixed Maturities Equity Securities	91,100,000			91,100,000
Fixed Maturities Equity Securities Amortized Cost	100,000,000			100,000,000
Short Term Investments, Medium Notes, Fair Value							454,800,000					485,600,000	370,800,000
Short Term Investments, Medium Notes, Amortized							504,600,000						423,900,000
Percentage of Fixed Income Investment portfolio invested in below investment grade or not rated	2.70%			2.70%			3.50%
Percentage Of Gross Unrealized Losses InFixed Income Securities Portfolio Related To Securities Below Investment Grade Or Not Rated	27.90%			27.90%			29.40%
Percentage Of Held To Maturity Securities Rated A Or Higher	98.10%
Investment Owned, Unrecognized Unrealized Appreciation									127,400,000
Available For Sale Securities Continuous Unrealized Loss Position Transferred Unamortized				113,900,000			119,000,000
Unrealized Gain (Loss) on Hybrid Instrument, Net				167,600,000			143,700,000
Available For Sale Securities Gross Unrealized Losses Considered As Temporarily Impaired Number Of Shares (in Shares)	1,996			1,996
Number Of Available For Sale Securities Held In Portfolio (in Shares)	7,197			7,197
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	20,700,000			20,700,000
Held To Maturity Securities Gross Unrealized Losses Considered As Temporarily Impaired Number Of Shares (in Shares)	43			43
Number Of Held To Maturity Securities Held In Portfolio (in Shares)	213			213
Available-for-sale Securities, Gross Realized Losses				944,700,000																									1,400,000,000
Available-for-sale Securities	27,781,192,000			27,781,192,000			27,677,553,000			398,600,000				1,300,000,000	163,300,000	358,700,000	270,000,000												226,900,000	297,900,000	205,100,000		80,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value										34,900,000				193,900,000	91,500,000					700,000			57,100,000	22,500,000					764,900,000	6,600,000,000	671,900,000	8,600,000,000	41,000,000
Held-to-maturity Securities	2,725,623,000			2,725,623,000			2,728,335,000										102,500,000
Percentage of Impairment on Amortized costs of Available for sale securities																																	50.00%
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount	66,800,000																	32,800,000			6,500,000	6,100,000
Impairments Related To Currency Losses	21,400,000
Credit loss impairments previously recognized on securities which matured, paid down, prepaid	(16,448,000)		(36,046,000)	(180,475,000)		(74,024,000)												8,300,000	120,900,000
Proceeds From Sale Of Interests In Affiliate Investments				35,000,000
Gain Loss On Sale Of Interests In Affiliate Investments				$ 25,300,000

[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 $485.6 million and $757.8 million at September 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 $167.6 million and $143.7 million at September 30, 2011 and December 31, 2010, respectively.