Investments (Details) - Analysis of continual unrealized loss position (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
HTM securities
Less than 12 months Fair Value	$ 2,126	$ 244,193
Less than 12 month Gross Unrealized Losses	(182)	(9,322)
Equal to or greater than 12 months Fair Value	59,303	81,779
Equal to or greater than 12 months Gross Unrealized Losses	(5,847)	(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	1,046,125	2,375,175
Less than 12 month Gross Unrealized Losses	(12,585)	(88,253)
Equal to or greater than 12 months Fair Value	2,318,539	2,782,909
Equal to or greater than 12 months Gross Unrealized Losses	(378,747)	(688,309)
U.S. Government and Government - Related/Supported [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	248,665	289,260
Less than 12 month Gross Unrealized Losses	(2,246)	(332)
Equal to or greater than 12 months Fair Value	25,692	43,622
Equal to or greater than 12 months Gross Unrealized Losses	(1,478)	(3,984)
Corporate
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	178,324	1,078,664
Less than 12 month Gross Unrealized Losses	(4,239)	(42,151)
Equal to or greater than 12 months Fair Value	821,415	1,185,535
Equal to or greater than 12 months Gross Unrealized Losses	(119,408)	(243,683)
HTM securities
Less than 12 months Fair Value	890	147,836
Less than 12 month Gross Unrealized Losses	(31)	(7,770)
Equal to or greater than 12 months Fair Value	41,547	62,343
Equal to or greater than 12 months Gross Unrealized Losses	(2,901)	(6,663)
RMBS - Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	145,444	310,318
Less than 12 month Gross Unrealized Losses	(1,698)	(849)
Equal to or greater than 12 months Fair Value	14,888	36,960
Equal to or greater than 12 months Gross Unrealized Losses	(1,040)	(3,000)
RMBS - Non-Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	10,119	106,294
Less than 12 month Gross Unrealized Losses	(200)	(31,714)
Equal to or greater than 12 months Fair Value	466,064	449,138
Equal to or greater than 12 months Gross Unrealized Losses	(107,557)	(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	90,884	69,109
Less than 12 month Gross Unrealized Losses	(485)	(2,716)
Equal to or greater than 12 months Fair Value	24,571	39,444
Equal to or greater than 12 months Gross Unrealized Losses	(3,502)	(6,837)
CDO
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	13,919	3,357
Less than 12 month Gross Unrealized Losses	(2,242)	(2,261)
Equal to or greater than 12 months Fair Value	655,280	636,362
Equal to or greater than 12 months Gross Unrealized Losses	(116,664)	(189,456)
Other asset-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	64,400	227,098
Less than 12 month Gross Unrealized Losses	(192)	(3,324)
Equal to or greater than 12 months Fair Value	105,520	161,312
Equal to or greater than 12 months Gross Unrealized Losses	(19,294)	(28,467)
HTM securities
Less than 12 months Fair Value	1,236	7,743
Less than 12 month Gross Unrealized Losses	(151)	(314)
Equal to or greater than 12 months Fair Value	8,273	1,106
Equal to or greater than 12 months Gross Unrealized Losses	(144)	(6)
U.S. States and political subdivisions of the States [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	13,136	25,309
Less than 12 month Gross Unrealized Losses	(137)	(199)
Equal to or greater than 12 months Fair Value	13,788	27,646
Equal to or greater than 12 months Gross Unrealized Losses	(1,516)	(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	281,234	265,766
Less than 12 month Gross Unrealized Losses	(1,146)	(4,707)
Equal to or greater than 12 months Fair Value	191,321	202,890
Equal to or greater than 12 months Gross Unrealized Losses	(8,288)	(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	9,483	18,330
Equal to or greater than 12 months Gross Unrealized Losses	(2,802)	(4,744)
Equity Securities [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	272,543	361,585
Less than 12 month Gross Unrealized Losses	(5,253)	(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