Investments - Analysis of continual unrealized loss position (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
HTM securities
Less than 12 months Fair Value	$ 51,060	$ 5,807
Less than 12 months Gross Unrealized Losses	(2,095)	(198)
Equal to or greater than 12 months Fair Value	12,546	33,523
Equal to or greater than 12 months Gross Unrealized Losses	(1,166)	(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	8,357,783	2,007,085
Less than 12 months Gross Unrealized Losses	(265,764)	(22,424)
Equal to or greater than 12 months Fair Value	1,680,520	2,033,491
Equal to greater than 12 months Gross Unrealized Losses	(173,418)	(260,326)
U.S. Government and Government - Related/Supported
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	715,162	307,879
Less than 12 months Gross Unrealized Losses	(19,021)	(2,847)
Equal to or greater than 12 months Fair Value	12,062	9,951
Equal to greater than 12 months Gross Unrealized Losses	(735)	(471)
Corporate
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	2,220,584	476,454
Less than 12 months Gross Unrealized Losses	(69,327)	(10,603)
Equal to or greater than 12 months Fair Value	457,493	607,796
Equal to greater than 12 months Gross Unrealized Losses	(55,194)	(68,387)
HTM securities
Less than 12 months Fair Value	51,060	4,568
Less than 12 months Gross Unrealized Losses	(2,095)	(31)
Equal to or greater than 12 months Fair Value	715	23,005
Equal to or greater than 12 months Gross Unrealized Losses	(27)	(763)
RMBS - Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	1,879,061	578,823
Less than 12 months Gross Unrealized Losses	(53,263)	(4,541)
Equal to or greater than 12 months Fair Value	24,389	11,135
Equal to greater than 12 months Gross Unrealized Losses	(1,671)	(994)
RMBS - Non-Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	24,369	6,674
Less than 12 months Gross Unrealized Losses	(1,199)	(450)
Equal to or greater than 12 months Fair Value	256,719	448,555
Equal to greater than 12 months Gross Unrealized Losses	(39,018)	(76,418)
CMBS
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	275,319	92,899
Less than 12 months Gross Unrealized Losses	(11,066)	(666)
Equal to or greater than 12 months Fair Value	18,786	23,580
Equal to greater than 12 months Gross Unrealized Losses	(3,392)	(3,622)
CDO
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	0	243
Less than 12 months Gross Unrealized Losses	0	(1)
Equal to or greater than 12 months Fair Value	685,761	694,351
Equal to greater than 12 months Gross Unrealized Losses	(48,345)	(87,155)
Other asset-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	273,757	111,431
Less than 12 months Gross Unrealized Losses	(3,073)	(531)
Equal to or greater than 12 months Fair Value	92,997	93,388
Equal to greater than 12 months Gross Unrealized Losses	(10,107)	(14,904)
HTM securities
Less than 12 months Fair Value	0	1,239
Less than 12 months Gross Unrealized Losses	0	(167)
Equal to or greater than 12 months Fair Value	1,114	0
Equal to or greater than 12 months Gross Unrealized Losses	(214)	0
U.S. States and political subdivisions of the States
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	398,984	77,273
Less than 12 months Gross Unrealized Losses	(23,965)	(1,407)
Equal to or greater than 12 months Fair Value	11,241	12,851
Equal to greater than 12 months Gross Unrealized Losses	(760)	(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	2,570,547	355,409
Less than 12 months Gross Unrealized Losses	(84,850)	(1,378)
Equal to or greater than 12 months Fair Value	121,072	131,884
Equal to greater than 12 months Gross Unrealized Losses	(14,196)	(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	10,717	10,518
Equal to or greater than 12 months Gross Unrealized Losses	(925)	(1,720)
Equities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	282,531	615
Less than 12 months Gross Unrealized Losses	(21,907)	(62)
Equal to or greater than 12 months Fair Value	0	0
Equal to greater than 12 months Gross Unrealized Losses	$ 0	$ 0