Investments - Analysis of continual unrealized loss position (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
HTM securities
Less than 12 months Fair Value	$ 32,424	$ 5,807
Less than 12 months Gross Unrealized Losses	(1,410)	(198)
Equal to or greater than 12 months Fair Value	11,479	33,523
Equal to or greater than 12 months Gross Unrealized Losses	(1,376)	(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	7,065,164	2,007,085
Less than 12 months Gross Unrealized Losses	(205,469)	(22,424)
Equal to or greater than 12 months Fair Value	1,828,487	2,033,491
Equal to greater than 12 months Gross Unrealized Losses	(179,849)	(260,326)
U.S. Government and Government - Related/Supported
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	731,583	307,879
Less than 12 months Gross Unrealized Losses	(18,079)	(2,847)
Equal to or greater than 12 months Fair Value	21,489	9,951
Equal to greater than 12 months Gross Unrealized Losses	(929)	(471)
Corporate
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	1,962,868	476,454
Less than 12 months Gross Unrealized Losses	(51,810)	(10,603)
Equal to or greater than 12 months Fair Value	478,878	607,796
Equal to greater than 12 months Gross Unrealized Losses	(54,190)	(68,387)
HTM securities
Less than 12 months Fair Value	32,424	4,568
Less than 12 months Gross Unrealized Losses	(1,410)	(31)
Equal to or greater than 12 months Fair Value	632	23,005
Equal to or greater than 12 months Gross Unrealized Losses	(26)	(763)
RMBS - Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	1,457,826	578,823
Less than 12 months Gross Unrealized Losses	(36,568)	(4,541)
Equal to or greater than 12 months Fair Value	164,046	11,135
Equal to greater than 12 months Gross Unrealized Losses	(11,790)	(994)
RMBS - Non-Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	29,777	6,674
Less than 12 months Gross Unrealized Losses	(1,257)	(450)
Equal to or greater than 12 months Fair Value	244,218	448,555
Equal to greater than 12 months Gross Unrealized Losses	(37,919)	(76,418)
CMBS
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	287,314	92,899
Less than 12 months Gross Unrealized Losses	(9,216)	(666)
Equal to or greater than 12 months Fair Value	38,653	23,580
Equal to greater than 12 months Gross Unrealized Losses	(4,055)	(3,622)
CDO
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	29,541	243
Less than 12 months Gross Unrealized Losses	(175)	(1)
Equal to or greater than 12 months Fair Value	622,338	694,351
Equal to greater than 12 months Gross Unrealized Losses	(49,648)	(87,155)
Other asset-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	222,044	111,431
Less than 12 months Gross Unrealized Losses	(1,821)	(531)
Equal to or greater than 12 months Fair Value	96,251	93,388
Equal to greater than 12 months Gross Unrealized Losses	(8,345)	(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	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	457,908	77,273
Less than 12 months Gross Unrealized Losses	(27,384)	(1,407)
Equal to or greater than 12 months Fair Value	8,812	12,851
Equal to greater than 12 months Gross Unrealized Losses	(700)	(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	1,886,303	355,409
Less than 12 months Gross Unrealized Losses	(59,159)	(1,378)
Equal to or greater than 12 months Fair Value	153,802	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	10,847	10,518
Equal to or greater than 12 months Gross Unrealized Losses	(1,350)	(1,720)
Equities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	216,631	615
Less than 12 months Gross Unrealized Losses	(17,021)	(62)
Equal to or greater than 12 months Fair Value	0	0
Equal to greater than 12 months Gross Unrealized Losses	$ 0	$ 0