Investments - Analysis of continual unrealized loss position (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
HTM securities
Less than 12 months Fair Value	$ 0	$ 46,034
Less than 12 months Gross Unrealized Losses	0	(941)
Equal to or greater than 12 months Fair Value	0	12,536
Equal to or greater than 12 months Gross Unrealized Losses	0	(669)
Fixed Maturities And Short Term Investments Available For Sale [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	3,641,618	8,871,868
Less than 12 months Gross Unrealized Losses	(42,479)	(230,109)
Equal to or greater than 12 months Fair Value	3,401,604	2,117,444
Equal to greater than 12 months Gross Unrealized Losses	(140,269)	(189,457)
U.S. Government and Government - Related/Supported
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value		1,333,704
Less than 12 months Gross Unrealized Losses		(30,474)
Equal to or greater than 12 months Fair Value		44,158
Equal to greater than 12 months Gross Unrealized Losses		(3,614)
Corporate
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value		2,756,235
Less than 12 months Gross Unrealized Losses		(59,497)
Equal to or greater than 12 months Fair Value		513,106
Equal to greater than 12 months Gross Unrealized Losses		(50,160)
HTM securities
Less than 12 months Fair Value		46,034
Less than 12 months Gross Unrealized Losses		(941)
Equal to or greater than 12 months Fair Value		642
Equal to or greater than 12 months Gross Unrealized Losses		(27)
RMBS - Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value		1,485,261
Less than 12 months Gross Unrealized Losses		(50,362)
Equal to or greater than 12 months Fair Value		169,704
Equal to greater than 12 months Gross Unrealized Losses		(11,715)
RMBS - Non-Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value		14,204
Less than 12 months Gross Unrealized Losses		(604)
Equal to or greater than 12 months Fair Value		240,946
Equal to greater than 12 months Gross Unrealized Losses		(30,522)
CMBS
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value		432,820
Less than 12 months Gross Unrealized Losses		(6,816)
Equal to or greater than 12 months Fair Value		107,192
Equal to greater than 12 months Gross Unrealized Losses		(8,957)
CDO
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value		58,239
Less than 12 months Gross Unrealized Losses		(217)
Equal to or greater than 12 months Fair Value		574,613
Equal to greater than 12 months Gross Unrealized Losses		(42,717)
Other asset-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value		196,639
Less than 12 months Gross Unrealized Losses		(2,149)
Equal to or greater than 12 months Fair Value		96,528
Equal to greater than 12 months Gross Unrealized Losses		(6,691)
U.S. States and political subdivisions of the States
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value		463,974
Less than 12 months Gross Unrealized Losses		(23,124)
Equal to or greater than 12 months Fair Value		64,324
Equal to greater than 12 months Gross Unrealized Losses		(7,646)
Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value		2,130,792
Less than 12 months Gross Unrealized Losses		(56,866)
Equal to or greater than 12 months Fair Value		306,873
Equal to greater than 12 months Gross Unrealized Losses		(27,435)
HTM securities
Less than 12 months Fair Value		0
Less than 12 months Gross Unrealized Losses		0
Equal to or greater than 12 months Fair Value		11,894
Equal to or greater than 12 months Gross Unrealized Losses		(642)
Equities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	226,398	155,453
Less than 12 months Gross Unrealized Losses	(14,962)	(17,470)
Equal to or greater than 12 months Fair Value	0	0
Equal to greater than 12 months Gross Unrealized Losses	0	0
Excluding Life Funds Withheld Assets | Fixed Maturities And Short Term Investments Available For Sale [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	3,641,618
Less than 12 months Gross Unrealized Losses	(42,479)
Equal to or greater than 12 months Fair Value	3,401,604
Equal to greater than 12 months Gross Unrealized Losses	(140,269)
Excluding Life Funds Withheld Assets | U.S. Government and Government - Related/Supported
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	626,407
Less than 12 months Gross Unrealized Losses	(2,546)
Equal to or greater than 12 months Fair Value	426,284
Equal to greater than 12 months Gross Unrealized Losses	(9,761)
Excluding Life Funds Withheld Assets | Corporate
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	1,622,006
Less than 12 months Gross Unrealized Losses	(20,672)
Equal to or greater than 12 months Fair Value	712,538
Equal to greater than 12 months Gross Unrealized Losses	(26,284)
Excluding Life Funds Withheld Assets | RMBS - Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	208,062
Less than 12 months Gross Unrealized Losses	(899)
Equal to or greater than 12 months Fair Value	625,783
Equal to greater than 12 months Gross Unrealized Losses	(20,830)
Excluding Life Funds Withheld Assets | RMBS - Non-Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	40,535
Less than 12 months Gross Unrealized Losses	(635)
Equal to or greater than 12 months Fair Value	212,278
Equal to greater than 12 months Gross Unrealized Losses	(18,325)
Excluding Life Funds Withheld Assets | CMBS
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	118,560
Less than 12 months Gross Unrealized Losses	(502)
Equal to or greater than 12 months Fair Value	184,427
Equal to greater than 12 months Gross Unrealized Losses	(6,991)
Excluding Life Funds Withheld Assets | CDO
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	173,729
Less than 12 months Gross Unrealized Losses	(1,363)
Equal to or greater than 12 months Fair Value	438,161
Equal to greater than 12 months Gross Unrealized Losses	(24,976)
Excluding Life Funds Withheld Assets | Other asset-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	73,985
Less than 12 months Gross Unrealized Losses	(1,415)
Equal to or greater than 12 months Fair Value	66,238
Equal to greater than 12 months Gross Unrealized Losses	(3,024)
Excluding Life Funds Withheld Assets | U.S. States and political subdivisions of the States
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	40,930
Less than 12 months Gross Unrealized Losses	(453)
Equal to or greater than 12 months Fair Value	138,328
Equal to greater than 12 months Gross Unrealized Losses	(2,374)
Excluding Life Funds Withheld Assets | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	737,404
Less than 12 months Gross Unrealized Losses	(13,994)
Equal to or greater than 12 months Fair Value	597,567
Equal to greater than 12 months Gross Unrealized Losses	(27,704)
Life Funds Withheld Assets | Fixed Maturities And Short Term Investments Available For Sale [Member]
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	0
Less than 12 months Gross Unrealized Losses	0
Equal to or greater than 12 months Fair Value	0
Equal to greater than 12 months Gross Unrealized Losses	$ 0