Investments (Details) - Analysis of continual unrealized loss position (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fixed Maturities And Short Term Investments Available For Sale [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	$ 2,007,085	$ 2,375,175
Less than 12 month Gross Unrealized Losses	(22,424)	(88,253)
Equal to or greater than 12 months Fair Value	2,033,491	2,782,909
Equal to or greater than 12 months Gross Unrealized Losses	(260,326)	(688,309)
HTM securities
Less than 12 months Fair Value	5,807	244,193
Less than 12 month Gross Unrealized Losses	(198)	(9,322)
Equal to or greater than 12 months Fair Value	33,523	81,779
Equal to or greater than 12 months Gross Unrealized Losses	(2,483)	(11,413)
U.S. Government and Government - Related/Supported [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	307,879	289,260
Less than 12 month Gross Unrealized Losses	(2,847)	(332)
Equal to or greater than 12 months Fair Value	9,951	43,622
Equal to or greater than 12 months Gross Unrealized Losses	(471)	(3,984)
Corporate
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	476,454	1,078,664
Less than 12 month Gross Unrealized Losses	(10,603)	(42,151)
Equal to or greater than 12 months Fair Value	607,796	1,185,535
Equal to or greater than 12 months Gross Unrealized Losses	(68,387)	(243,683)
HTM securities
Less than 12 months Fair Value	4,568	147,836
Less than 12 month Gross Unrealized Losses	(31)	(7,770)
Equal to or greater than 12 months Fair Value	23,005	62,343
Equal to or greater than 12 months Gross Unrealized Losses	(763)	(6,663)
RMBS - Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	578,823	310,318
Less than 12 month Gross Unrealized Losses	(4,541)	(849)
Equal to or greater than 12 months Fair Value	11,135	36,960
Equal to or greater than 12 months Gross Unrealized Losses	(994)	(3,000)
RMBS - Non-Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	6,674	106,294
Less than 12 month Gross Unrealized Losses	(450)	(31,714)
Equal to or greater than 12 months Fair Value	448,555	449,138
Equal to or greater than 12 months Gross Unrealized Losses	(76,418)	(197,695)
HTM securities
Less than 12 months Fair Value		9,372
Less than 12 month Gross Unrealized Losses		(32)
Equal to or greater than 12 months Fair Value		0
Equal to or greater than 12 months Gross Unrealized Losses		0
CMBS
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	92,899	69,109
Less than 12 month Gross Unrealized Losses	(666)	(2,716)
Equal to or greater than 12 months Fair Value	23,580	39,444
Equal to or greater than 12 months Gross Unrealized Losses	(3,622)	(6,837)
CDO
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	243	3,357
Less than 12 month Gross Unrealized Losses	(1)	(2,261)
Equal to or greater than 12 months Fair Value	694,351	636,362
Equal to or greater than 12 months Gross Unrealized Losses	(87,155)	(189,456)
Other asset-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	111,431	227,098
Less than 12 month Gross Unrealized Losses	(531)	(3,324)
Equal to or greater than 12 months Fair Value	93,388	161,312
Equal to or greater than 12 months Gross Unrealized Losses	(14,904)	(28,467)
HTM securities
Less than 12 months Fair Value	1,239	7,743
Less than 12 month Gross Unrealized Losses	(167)	(314)
Equal to or greater than 12 months Fair Value	0	1,106
Equal to or greater than 12 months Gross Unrealized Losses	0	(6)
U.S. States and political subdivisions of the States [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	77,273	25,309
Less than 12 month Gross Unrealized Losses	(1,407)	(199)
Equal to or greater than 12 months Fair Value	12,851	27,646
Equal to or greater than 12 months Gross Unrealized Losses	(1,539)	(2,021)
Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	355,409	265,766
Less than 12 month Gross Unrealized Losses	(1,378)	(4,707)
Equal to or greater than 12 months Fair Value	131,884	202,890
Equal to or greater than 12 months Gross Unrealized Losses	(6,836)	(13,166)
HTM securities
Less than 12 months Fair Value	0	79,242
Less than 12 month Gross Unrealized Losses	0	(1,206)
Equal to or greater than 12 months Fair Value	10,518	18,330
Equal to or greater than 12 months Gross Unrealized Losses	(1,720)	(4,744)
Equity Securities [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	615	361,585
Less than 12 month Gross Unrealized Losses	(62)	(40,435)
Equal to or greater than 12 months Fair Value	0	0
Equal to or greater than 12 months Gross Unrealized Losses	$ 0	$ 0