Investments (Detail) (USD $)	3 Months Ended			6 Months Ended		12 Months Ended						3 Months Ended		3 Months Ended														54 Months Ended
	Sep. 30, 2011	Jun. 30, 2011		Jun. 30, 2011		Dec. 31, 2010		Dec. 30, 2010	Aug. 01, 2010	Nov. 01, 2009	Jun. 30, 2011 Tier One And Upper Tier Two Securities [Member] Life Operations Investment Portfolio [Member]	Jun. 30, 2011 Non AgencyRMBS [Member]	Jun. 30, 2011 Non AgencyRMBS [Member] Structured Credit Securities [Member]	Jun. 30, 2011 Corporate Securities Including Medium Term Notes Backed Primarily ByInvestment Grade Eurpoean Credit [Member]	Jun. 30, 2011 Medium-term Notes [Member] Non Life Fixed Income Portfolios [Member]	Jun. 30, 2011 CoreCDO Holdings Portfolio [Member] Structured Credit Securities [Member]	Jun. 30, 2011 CMBS Holdings Portfolio [Member] Structured Credit Securities [Member]	Dec. 31, 2010 Continual Unrealized Loss Position [Member]	Jun. 30, 2011 Continual Unrealized Loss Position [Member]	Dec. 31, 2010 Continual Unrealized Loss Position [Member]	Jun. 30, 2011 Corporate Financial Instituion [Member] Life Operations Investment Portfolio [Member]	Jun. 30, 2011 Corporate Financial Instituion [Member] Non Life Fixed Income Portfolios [Member]	Jun. 30, 2011 Hypothetical Interest And Credit Rate Increase [Member] Life Operations Investment Portfolio [Member]	Jun. 30, 2011 Government Related [Member]	Dec. 31, 2010 Government Related [Member]	Jun. 30, 2011 US Government Agencies Debt Securities [Member]	Dec. 31, 2010 US Government Agencies Debt Securities [Member]	Jun. 30, 2011 Residential Mortgage Backed Securities Non Agency [Member]	Jun. 30, 2011 Life Operations Investment Portfolio [Member]	Jun. 30, 2011 Topical Asset Portfolio [Member]	Jun. 30, 2011 Non Life Fixed Income Portfolios [Member]	Jun. 30, 2011 Structured Credit Securities [Member]
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis		$ 25,105,488,000	[1]	$ 25,105,488,000	[1]	$ 25,714,886,000	[1]																	$ 2,014,900,000	$ 2,101,000,000	$ 699,600,000	$ 1,019,200,000
Available-for-sale Securities, Debt Securities		25,151,686,000	[1]	25,151,686,000	[1]	25,544,179,000	[1]																	2,025,000,000	2,131,200,000	726,100,000	1,072,600,000
Short Term Investments, Medium Notes, Fair Value		491,300,000		491,300,000		454,800,000													491,300,000	370,800,000
Short Term Investments, Medium Notes, Amortized		503,900,000		503,900,000		504,600,000														423,900,000								503,900,000
Fixed Maturities, Tier One And Two Securities, Fair Value		641,900,000		641,900,000														757,800,000	641,900,000
Fixed Maturities, Tier One and Two Securities, Amortized		746,100,000		746,100,000														883,000,000	746,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		819,500,000		819,500,000									51,700,000		23,400,000	21,600,000	2,300,000				111,900,000	71,100,000						220,300,000	250,100,000	159,500,000	169,700,000	75,600,000
Percentage of Fixed Income Investment portfolio invested in below investment grade or not rated		2.90%		2.90%		3.50%
Percentage Of Gross Unrealized Losses InFixed Income Securities Portfolio Related To Securities Below Investment Grade Or Not Rated		27.60%		27.60%		29.40%
Unrealized Gain (Loss) on Hybrid Instrument, Net				104,200,000		143,700,000
Available-for-sale Securities, Gross Realized Losses				819,500,000																								1,400,000,000
Available-for-sale Securities		28,322,204,000		28,322,204,000		27,677,553,000					489,700,000				461,500,000						1,500,000,000		260,200,000					858,500,000	6,800,000,000	733,800,000	8,800,000	39,600,000
Held-to-maturity Securities		2,843,371,000		2,843,371,000		2,728,335,000																	101,800,000
Percentage of Impairment on Amortized costs of Available for sale securities																																50.00%
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount		27,200,000										14,000,000		11,100,000
Impairments Related To Currency Losses		2,100,000
Percentage Of Held To Maturity Securities Rated A Or Higher		99.20%
Investment Owned, Unrecognized Unrealized Appreciation									76,200,000	51,200,000
Available For Sale Securities Continuous Unrealized Loss Position Transferred Unamortized				122,000,000		119,000,000
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value		48,300,000		48,300,000				24,700,000
Cumulative Effect on Retained Earnings, before Tax		31,900,000		31,900,000
Proceeds From Sale Of Interests In Affiliate Investments				35,000,000
Gain Loss On Sale Of Interests In Affiliate Investments	$ 21,800,000

[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.