Investments (Details) - Analysis of continual unrealized loss position (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
HTM securities
Less than 12 months Fair Value	$ 57,559	$ 244,193
Less than 12 month Gross Unrealized Losses	(924)	(9,322)
Equal to or greater than 12 months Fair Value	58,342	81,779
Equal to or greater than 12 months Gross Unrealized Losses	(11,576)	(11,413)
Fixed Maturities And Short Term Investments Available For Sale [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	1,295,169	2,375,175
Less than 12 month Gross Unrealized Losses	(26,298)	(88,253)
Equal to or greater than 12 months Fair Value	2,368,776	2,782,909
Equal to or greater than 12 months Gross Unrealized Losses	(522,669)	(688,309)
U.S. Government and Government - Related/Supported [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	311,939	289,260
Less than 12 month Gross Unrealized Losses	(1,902)	(332)
Equal to or greater than 12 months Fair Value	10,518	43,622
Equal to or greater than 12 months Gross Unrealized Losses	(1,014)	(3,984)
Corporate
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	422,004	1,078,664
Less than 12 month Gross Unrealized Losses	(12,834)	(42,151)
Equal to or greater than 12 months Fair Value	903,366	1,185,535
Equal to or greater than 12 months Gross Unrealized Losses	(160,988)	(243,683)
HTM securities
Less than 12 months Fair Value	23,418	147,836
Less than 12 month Gross Unrealized Losses	(374)	(7,770)
Equal to or greater than 12 months Fair Value	49,617	62,343
Equal to or greater than 12 months Gross Unrealized Losses	(8,473)	(6,663)
RMBS - Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	100,286	310,318
Less than 12 month Gross Unrealized Losses	(1,496)	(849)
Equal to or greater than 12 months Fair Value	14,405	36,960
Equal to or greater than 12 months Gross Unrealized Losses	(980)	(3,000)
RMBS - Non-Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	29,190	106,294
Less than 12 month Gross Unrealized Losses	(1,745)	(31,714)
Equal to or greater than 12 months Fair Value	462,704	449,138
Equal to or greater than 12 months Gross Unrealized Losses	(163,843)	(197,695)
HTM securities
Less than 12 months Fair Value	4,996	9,372
Less than 12 month Gross Unrealized Losses	(15)	(32)
Equal to or greater than 12 months Fair Value	0	0
Equal to or greater than 12 months Gross Unrealized Losses	0	0
CMBS
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	13,945	69,109
Less than 12 month Gross Unrealized Losses	(281)	(2,716)
Equal to or greater than 12 months Fair Value	30,996	39,444
Equal to or greater than 12 months Gross Unrealized Losses	(3,787)	(6,837)
CDO
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	13,727	3,357
Less than 12 month Gross Unrealized Losses	(2,651)	(2,261)
Equal to or greater than 12 months Fair Value	627,018	636,362
Equal to or greater than 12 months Gross Unrealized Losses	(151,444)	(189,456)
Other asset-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	97,511	227,098
Less than 12 month Gross Unrealized Losses	(972)	(3,324)
Equal to or greater than 12 months Fair Value	139,949	161,312
Equal to or greater than 12 months Gross Unrealized Losses	(28,329)	(28,467)
HTM securities
Less than 12 months Fair Value	8,972	7,743
Less than 12 month Gross Unrealized Losses	(464)	(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	10,227	25,309
Less than 12 month Gross Unrealized Losses	(121)	(199)
Equal to or greater than 12 months Fair Value	16,029	27,646
Equal to or greater than 12 months Gross Unrealized Losses	(1,660)	(2,021)
Non US Sovereign Government Supranational And Government Related [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	296,340	265,766
Less than 12 month Gross Unrealized Losses	(4,296)	(4,707)
Equal to or greater than 12 months Fair Value	163,791	202,890
Equal to or greater than 12 months Gross Unrealized Losses	(10,624)	(13,166)
HTM securities
Less than 12 months Fair Value	20,173	79,242
Less than 12 month Gross Unrealized Losses	(71)	(1,206)
Equal to or greater than 12 months Fair Value	8,725	18,330
Equal to or greater than 12 months Gross Unrealized Losses	(3,103)	(4,744)
Equity Securities [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	422,204	361,585
Less than 12 month Gross Unrealized Losses	(27,871)	(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