Investments - Analysis of continual unrealized loss position (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
HTM securities
Less than 12 months Fair Value	$ 1,913	$ 5,807
Less than 12 months Gross Unrealized Losses	(179)	(198)
Equal to or greater than 12 months Fair Value	21,358	33,523
Equal to or greater than 12 months Gross Unrealized Losses	(1,770)	(2,483)
Fixed Maturities And Short Term Investments Available For Sale [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	3,350,375	2,007,085
Less than 12 months Gross Unrealized Losses	(47,563)	(22,424)
Equal to or greater than 12 months Fair Value	1,890,992	2,033,491
Equal to greater than 12 months Gross Unrealized Losses	(199,944)	(260,326)
U.S. Government and Government - Related/Supported
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	197,384	307,879
Less than 12 months Gross Unrealized Losses	(1,644)	(2,847)
Equal to or greater than 12 months Fair Value	15,871	9,951
Equal to greater than 12 months Gross Unrealized Losses	(1,066)	(471)
Corporate
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	785,879	476,454
Less than 12 months Gross Unrealized Losses	(11,646)	(10,603)
Equal to or greater than 12 months Fair Value	514,156	607,796
Equal to greater than 12 months Gross Unrealized Losses	(54,098)	(68,387)
HTM securities
Less than 12 months Fair Value	720	4,568
Less than 12 months Gross Unrealized Losses	(23)	(31)
Equal to or greater than 12 months Fair Value	10,720	23,005
Equal to or greater than 12 months Gross Unrealized Losses	(884)	(763)
RMBS - Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	942,759	578,823
Less than 12 months Gross Unrealized Losses	(8,722)	(4,541)
Equal to or greater than 12 months Fair Value	18,249	11,135
Equal to greater than 12 months Gross Unrealized Losses	(1,641)	(994)
RMBS - Non-Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	9,197	6,674
Less than 12 months Gross Unrealized Losses	(664)	(450)
Equal to or greater than 12 months Fair Value	405,232	448,555
Equal to greater than 12 months Gross Unrealized Losses	(52,479)	(76,418)
CMBS
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	205,453	92,899
Less than 12 months Gross Unrealized Losses	(2,116)	(666)
Equal to or greater than 12 months Fair Value	19,846	23,580
Equal to greater than 12 months Gross Unrealized Losses	(3,254)	(3,622)
CDO
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	99	243
Less than 12 months Gross Unrealized Losses	0	(1)
Equal to or greater than 12 months Fair Value	690,248	694,351
Equal to greater than 12 months Gross Unrealized Losses	(65,323)	(87,155)
Other asset-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	106,354	111,431
Less than 12 months Gross Unrealized Losses	(377)	(531)
Equal to or greater than 12 months Fair Value	93,772	93,388
Equal to greater than 12 months Gross Unrealized Losses	(9,122)	(14,904)
HTM securities
Less than 12 months Fair Value	1,193	1,239
Less than 12 months Gross Unrealized Losses	(156)	(167)
Equal to or greater than 12 months Fair Value	0	0
Equal to or greater than 12 months Gross Unrealized Losses	0	0
U.S. States and political subdivisions of the States
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	215,939	77,273
Less than 12 months Gross Unrealized Losses	(4,060)	(1,407)
Equal to or greater than 12 months Fair Value	10,264	12,851
Equal to greater than 12 months Gross Unrealized Losses	(688)	(1,539)
Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	887,311	355,409
Less than 12 months Gross Unrealized Losses	(18,334)	(1,378)
Equal to or greater than 12 months Fair Value	123,354	131,884
Equal to greater than 12 months Gross Unrealized Losses	(12,273)	(6,836)
HTM securities
Less than 12 months Fair Value	0	0
Less than 12 months Gross Unrealized Losses	0	0
Equal to or greater than 12 months Fair Value	10,638	10,518
Equal to or greater than 12 months Gross Unrealized Losses	(886)	(1,720)
Equities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	86,305	615
Less than 12 months Gross Unrealized Losses	(2,214)	(62)
Equal to or greater than 12 months Fair Value	0	0
Equal to greater than 12 months Gross Unrealized Losses	$ 0	$ 0