Investments (Detail) - Analysis of continual unrealized loss position (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012		Dec. 31, 2011
Fixed maturities -HTM
Less than 12 months Fair Value - HTM	$ 153,395		$ 244,193
Less than 12 month Gross Unrealized Losses - HTM	(4,027)		(9,322)
Equal to or greater than 12 months Fair Value - HTM	74,468		81,779
Equal to or greater than 12 months Gross Unrealized Losses - HTM	(5,170)		(11,413)
Corporate Debt Securities [Member]
Fixed maturities -HTM
Less than 12 months Fair Value - HTM	61,497	[1]	147,836	[2]
Less than 12 month Gross Unrealized Losses - HTM	(2,652)	[1]	(7,770)	[2]
Equal to or greater than 12 months Fair Value - HTM	54,404	[1]	62,343	[2]
Equal to or greater than 12 months Gross Unrealized Losses - HTM	(1,512)	[1]	(6,663)	[2]
Residential Mortgage-Backed Securities Non-Agency [Member]
Fixed maturities -HTM
Less than 12 months Fair Value - HTM	9,521		9,372
Less than 12 month Gross Unrealized Losses - HTM	(147)		(32)
Equal to or greater than 12 months Fair Value - HTM	0		0
Equal to or greater than 12 months Gross Unrealized Losses - HTM	0		0
Other Asset Backed Securities [Member]
Fixed maturities -HTM
Less than 12 months Fair Value - HTM	7,709	[1]	7,743	[2]
Less than 12 month Gross Unrealized Losses - HTM	(573)	[1]	(314)	[2]
Equal to or greater than 12 months Fair Value - HTM	0	[1]	1,106	[2]
Equal to or greater than 12 months Gross Unrealized Losses - HTM	0	[1]	(6)	[2]
Non US Sovereign Government Supranational And Government Related [Member]
Fixed maturities -HTM
Less than 12 months Fair Value - HTM	74,668		79,242
Less than 12 month Gross Unrealized Losses - HTM	(655)		(1,206)
Equal to or greater than 12 months Fair Value - HTM	20,064		18,330
Equal to or greater than 12 months Gross Unrealized Losses - HTM	(3,658)		(4,744)
US Government And Government Related [Member]
Fixed maturities and short-term investments - AFS
Less than 12 months Fair Value - AFS	478,076		289,260
Less than 12 month Gross Unrealized Losses - AFS	(3,042)		(332)
Equal to or greater than 12 months Fair Value - AFS	31,499		43,622
Equal to or greater than 12 months Gross Unrealized Losses - AFS	(4,183)		(3,984)
Corporate Debt Securities [Member]
Fixed maturities and short-term investments - AFS
Less than 12 months Fair Value - AFS	638,803	[1],[3],[4]	1,078,664	[2],[5],[6]
Less than 12 month Gross Unrealized Losses - AFS	(14,193)	[1],[3],[4]	(42,151)	[2],[5],[6]
Equal to or greater than 12 months Fair Value - AFS	1,084,471	[1],[3],[4]	1,185,535	[2],[5],[6]
Equal to or greater than 12 months Gross Unrealized Losses - AFS	(177,433)	[1],[3],[4]	(243,683)	[2],[5],[6]
Residential Mortgage-Backed Securities Agency [Member]
Fixed maturities and short-term investments - AFS
Less than 12 months Fair Value - AFS	1,081,932		310,318
Less than 12 month Gross Unrealized Losses - AFS	(5,053)		(849)
Equal to or greater than 12 months Fair Value - AFS	34,328		36,960
Equal to or greater than 12 months Gross Unrealized Losses - AFS	(3,630)		(3,000)
Residential Mortgage-Backed Securities Non-Agency [Member]
Fixed maturities and short-term investments - AFS
Less than 12 months Fair Value - AFS	36,572		106,294
Less than 12 month Gross Unrealized Losses - AFS	(3,466)		(31,714)
Equal to or greater than 12 months Fair Value - AFS	489,071		449,138
Equal to or greater than 12 months Gross Unrealized Losses - AFS	(180,507)		(197,695)
Commercial Mortgage Backed Securities [Member]
Fixed maturities and short-term investments - AFS
Less than 12 months Fair Value - AFS	41,017		69,109
Less than 12 month Gross Unrealized Losses - AFS	(618)		(2,716)
Equal to or greater than 12 months Fair Value - AFS	32,415		39,444
Equal to or greater than 12 months Gross Unrealized Losses - AFS	(4,407)		(6,837)
Collateralized Debt Obligations [Member]
Fixed maturities and short-term investments - AFS
Less than 12 months Fair Value - AFS	13,501		3,357
Less than 12 month Gross Unrealized Losses - AFS	(2,643)		(2,261)
Equal to or greater than 12 months Fair Value - AFS	622,684		636,362
Equal to or greater than 12 months Gross Unrealized Losses - AFS	(157,367)		(189,456)
Other Asset Backed Securities [Member]
Fixed maturities and short-term investments - AFS
Less than 12 months Fair Value - AFS	145,767	[1]	227,098	[2]
Less than 12 month Gross Unrealized Losses - AFS	(3,683)	[1]	(3,324)	[2]
Equal to or greater than 12 months Fair Value - AFS	163,694	[1]	161,312	[2]
Equal to or greater than 12 months Gross Unrealized Losses - AFS	(29,770)	[1]	(28,467)	[2]
US States and Political Subdivisions Debt Securities [Member]
Fixed maturities and short-term investments - AFS
Less than 12 months Fair Value - AFS	8,475		25,309
Less than 12 month Gross Unrealized Losses - AFS	(40)		(199)
Equal to or greater than 12 months Fair Value - AFS	16,045		27,646
Equal to or greater than 12 months Gross Unrealized Losses - AFS	(1,804)		(2,021)
Non US Sovereign Government Supranational And Government Related [Member]
Fixed maturities and short-term investments - AFS
Less than 12 months Fair Value - AFS	572,425		265,766
Less than 12 month Gross Unrealized Losses - AFS	(5,802)		(4,707)
Equal to or greater than 12 months Fair Value - AFS	166,060		202,890
Equal to or greater than 12 months Gross Unrealized Losses - AFS	(7,540)		(13,166)
Total Fixed Maturities And Short Term Investments Available For Sale [Member]
Fixed maturities and short-term investments - AFS
Less than 12 months Fair Value - AFS	3,016,568		2,375,175
Less than 12 month Gross Unrealized Losses - AFS	(38,540)		(88,253)
Equal to or greater than 12 months Fair Value - AFS	2,640,267		2,782,909
Equal to or greater than 12 months Gross Unrealized Losses - AFS	(566,641)		(688,309)
Equity Securities [Member]
Fixed maturities and short-term investments - AFS
Less than 12 months Fair Value - AFS	228,847	[7]	361,585	[8]
Less than 12 month Gross Unrealized Losses - AFS	(6,709)	[7]	(40,435)	[8]
Equal to or greater than 12 months Fair Value - AFS	0	[7]	0	[8]
Equal to or greater than 12 months Gross Unrealized Losses - AFS	$ 0	[7]	$ 0	[8]

[1] Covered Bonds within Fixed maturities and short-term investments - AFS with a fair value of $33.9 million and Covered Bonds within Fixed Maturities - HTM with a fair value of $7.7 million have been included within Other asset-backed securities to align the Company's classification to market indices. Covered Bonds were previously included in Corporate.

[2] Covered Bonds within Fixed maturities and short-term investments - AFS with a fair value of $44.7 million and Covered Bonds within Fixed Maturities - HTM with a fair value of $7.7 million have been included within Other asset-backed securities to align the Company's classification to market indices and to conform to current period presentation. Covered Bonds were previously included in Corporate.

[3] Included within Corporate are certain medium term notes supported primarily by pools of European investment grade credit with varying degrees of leverage. The notes, which are in a gross unrealized loss position, have a fair value of $198.7 million and an amortized cost of $220.0 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 $411.2 million and an amortized cost of $489.3 million at March 31, 2012.

[5] Included within Corporate are certain medium term notes supported primarily by pools of European investment grade credit with varying degrees of leverage. The notes, which are in a gross unrealized loss position, have a fair value of $266.0 million and an amortized cost of $297.7 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, 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 $386.1 million and an amortized cost of $494.9 million at December 31, 2011.

[7] Included within equity securities are investments in fixed income funds with a fair value of $98.0 million and an amortized cost of $100.0 million at March 31, 2012.

[8] Included within equity securities are investments in fixed income funds with a fair value of $91.6 million and an amortized cost of $100.0 million at December 31, 2011.