Investments (Detail) - Analysis of continual unrealized loss position (USD $) In Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2011		Dec. 31, 2010
Residential Mortgage-Backed Securities Agency [Member] | Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member]
Fixed maturities and short-term investments - AFS:
Fair Value of Fixed maturities and short term investments	$ 267,693		$ 280,390
Gross Unrealized Losses of Fixed maturities and short term investments	(1,362)		(6,736)	[1]
Residential Mortgage-Backed Securities Non-Agency [Member] | Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member]
Fixed maturities and short-term investments - AFS:
Fair Value of Fixed maturities and short term investments	112,395		40,052
Gross Unrealized Losses of Fixed maturities and short term investments	(31,710)		(2,574)	[1]
Fixed maturities – HTM:
Total fixed maturities – HTM	11,411		37,899
Total fixed maturities – HTM	(282)		(546)	[1]
Other Asset Backed Securities [Member] | Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member]
Fixed maturities and short-term investments - AFS:
Fair Value of Fixed maturities and short term investments	105,127		122,548
Gross Unrealized Losses of Fixed maturities and short term investments	(2,075)		(1,619)	[1]
Fixed maturities – HTM:
Total fixed maturities – HTM	1,118		232,673
Total fixed maturities – HTM	(16)		(1,410)	[1]
Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member]
Fixed maturities and short-term investments - AFS:
Fair Value of Fixed maturities and short term investments	2,848,785		4,265,457
Gross Unrealized Losses of Fixed maturities and short term investments	(120,384)		(154,359)	[1]
Total equity securities	370,535	[2]	158	[2]
Total equity securities	(55,829)	[2]	(53)	[1],[2]
Fixed maturities – HTM:
Total fixed maturities – HTM	272,388		1,212,106
Total fixed maturities – HTM	(18,554)		(24,691)	[1]
Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member] | US States and Political Subdivisions Debt Securities [Member]
Fixed maturities and short-term investments - AFS:
Fair Value of Fixed maturities and short term investments	46,831		734,893
Gross Unrealized Losses of Fixed maturities and short term investments	(288)		(30,033)	[1]
Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member] | US States and Political Subdivisions Debt Securities [Member] | Non US Sovereign Government Supranational And Government Related [Member]
Fixed maturities and short-term investments - AFS:
Fair Value of Fixed maturities and short term investments	298,381		459,686
Gross Unrealized Losses of Fixed maturities and short term investments	(5,338)		(5,116)	[1]
Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member] | Commercial Mortgage Backed Securities [Member]
Fixed maturities and short-term investments - AFS:
Fair Value of Fixed maturities and short term investments	103,931		46,419
Gross Unrealized Losses of Fixed maturities and short term investments	(4,297)		(2,472)	[1]
Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member] | Collateralized Debt Obligations [Member]
Fixed maturities and short-term investments - AFS:
Fair Value of Fixed maturities and short term investments	3,407		2,500
Gross Unrealized Losses of Fixed maturities and short term investments	(2,330)		(51)	[1]
Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member] | US Government And Government Related [Member]
Fixed maturities and short-term investments - AFS:
Fair Value of Fixed maturities and short term investments	344,783		307,082
Gross Unrealized Losses of Fixed maturities and short term investments	(1,977)		(25,482)	[1]
Fixed maturities – HTM:
Total fixed maturities – HTM			1,755
Total fixed maturities – HTM			(9)	[1]
Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member] | Corporate Debt Securities [Member]
Fixed maturities and short-term investments - AFS:
Fair Value of Fixed maturities and short term investments	1,566,237	[3],[4]	2,271,887	[5],[6]
Gross Unrealized Losses of Fixed maturities and short term investments	(71,007)	[3],[4]	(80,276)	[1],[5],[6]
Fixed maturities – HTM:
Total fixed maturities – HTM	236,991		764,397
Total fixed maturities – HTM	(16,006)		(16,325)	[1]
Investments Continuous Unrealized Loss Positition Less Than Twelve Months [Member] | Non US Sovereign Government Supranational And Government Related [Member]
Fixed maturities – HTM:
Total fixed maturities – HTM	22,868		175,382
Total fixed maturities – HTM	(2,250)		(6,401)	[1]
Residential Mortgage-Backed Securities Agency [Member] | Investments Continuous Unrealized Loss Position Over Twelve Months [Member]
Fixed maturities and short-term investments - AFS:
Fair Value of Fixed maturities and short term investments	32,129		34,186
Gross Unrealized Losses of Fixed maturities and short term investments	(3,429)	[1]	(1,913)	[1]
Residential Mortgage-Backed Securities Non-Agency [Member] | Investments Continuous Unrealized Loss Position Over Twelve Months [Member]
Fixed maturities and short-term investments - AFS:
Fair Value of Fixed maturities and short term investments	481,732		843,168
Gross Unrealized Losses of Fixed maturities and short term investments	(194,947)	[1]	(259,715)	[1]
Fixed maturities – HTM:
Total fixed maturities – HTM		[1]		[1]
Other Asset Backed Securities [Member] | Investments Continuous Unrealized Loss Position Over Twelve Months [Member]
Fixed maturities and short-term investments - AFS:
Fair Value of Fixed maturities and short term investments	168,434		226,946
Gross Unrealized Losses of Fixed maturities and short term investments	(23,931)	[1]	(33,546)	[1]
Fixed maturities – HTM:
Total fixed maturities – HTM		[1]		[1]
Investments Continuous Unrealized Loss Position Over Twelve Months [Member]
Fixed maturities and short-term investments - AFS:
Fair Value of Fixed maturities and short term investments	3,151,417		4,092,776
Gross Unrealized Losses of Fixed maturities and short term investments	(768,406)	[1]	(840,405)	[1]
Total equity securities		[2]		[2]
Total equity securities		[2]		[1],[2]
Fixed maturities – HTM:
Total fixed maturities – HTM	26,861
Total fixed maturities – HTM	(2,116)	[1]		[1]
Investments Continuous Unrealized Loss Position Over Twelve Months [Member] | US States and Political Subdivisions Debt Securities [Member]
Fixed maturities and short-term investments - AFS:
Fair Value of Fixed maturities and short term investments	48,906		40,907
Gross Unrealized Losses of Fixed maturities and short term investments	(1,855)	[1]	(5,452)	[1]
Investments Continuous Unrealized Loss Position Over Twelve Months [Member] | US States and Political Subdivisions Debt Securities [Member] | Non US Sovereign Government Supranational And Government Related [Member]
Fixed maturities and short-term investments - AFS:
Fair Value of Fixed maturities and short term investments	369,254		418,322
Gross Unrealized Losses of Fixed maturities and short term investments	(30,136)	[1]	(40,837)	[1]
Investments Continuous Unrealized Loss Position Over Twelve Months [Member] | Commercial Mortgage Backed Securities [Member]
Fixed maturities and short-term investments - AFS:
Fair Value of Fixed maturities and short term investments	38,618		69,475
Gross Unrealized Losses of Fixed maturities and short term investments	(7,119)	[1]	(15,967)	[1]
Investments Continuous Unrealized Loss Position Over Twelve Months [Member] | Collateralized Debt Obligations [Member]
Fixed maturities and short-term investments - AFS:
Fair Value of Fixed maturities and short term investments	657,586		715,295
Gross Unrealized Losses of Fixed maturities and short term investments	(202,882)	[1]	(197,535)	[1]
Investments Continuous Unrealized Loss Position Over Twelve Months [Member] | US Government And Government Related [Member]
Fixed maturities and short-term investments - AFS:
Fair Value of Fixed maturities and short term investments	47,632		117,394
Gross Unrealized Losses of Fixed maturities and short term investments	(4,203)	[1]	(10,417)	[1]
Fixed maturities – HTM:
Total fixed maturities – HTM		[1]		[1]
Investments Continuous Unrealized Loss Position Over Twelve Months [Member] | Corporate Debt Securities [Member]
Fixed maturities and short-term investments - AFS:
Fair Value of Fixed maturities and short term investments	1,307,126	[3],[4]	1,627,083	[5],[6]
Gross Unrealized Losses of Fixed maturities and short term investments	(299,904)	[1],[3],[4]	(275,023)	[1],[5],[6]
Fixed maturities – HTM:
Total fixed maturities – HTM	26,861
Total fixed maturities – HTM	(2,116)	[1]		[1]
Investments Continuous Unrealized Loss Position Over Twelve Months [Member] | Non US Sovereign Government Supranational And Government Related [Member]
Fixed maturities – HTM:
Total fixed maturities – HTM		[1]		[1]

[1] On securities impacted by the April 1, 2009 changes to OTTI values, length of time of impairment is measured from the point at which securities returned to a net unrealized loss position (i.e., from April 1, 2009).

[2] Included within equity securities are investments in fixed income funds with a fair value of $91.1 million and an amortized cost of $100.0 million at September 30, 2011.

[3] Included within Corporate are certain medium term notes supported primarily by pools of European credit with varying degrees of leverage. The notes, which are in a gross unrealized loss position, have a fair value of $429.2 million and an amortized cost of $460.0 million. These notes allow the investor to participate in cash flows of the underlying bonds including certain residual values, which could serve to either decrease or increase the ultimate values of these notes.

[4] Included within Corporate 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. These securities, which are in a gross unrealized loss position, have a fair value of $485.6 million and an amortized cost of $653.2 million at September 30, 2011.

[5] Included within Corporate are certain medium term notes supported primarily by pools of European credit with varying degrees of leverage. The notes have a fair value of $370.8 million and an amortized cost of $423.9 million. These notes allow the investor to participate in cash flows of the underlying bonds including certain residual values, which could serve to either decrease or increase the ultimate values of these notes.

[6] Included within Corporate 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. These securities have a fair value of $757.8 million and an amortized cost of $883.0 million at December 31, 2010.