Fair Value Measurements (Details) - Additional information about assets and liabilities measured at fair value on a recurring basis (USD $) In Thousands, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Derivative Contracts - Net
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period	$ 35,947		$ 49,645	$ 42,644	$ 39,195
Realized gains (losses)	0			0	0
Movement in unrealized gains (losses)	(885)		2,344	(7,582)	12,620
Purchases and issuances	0		0	0	0
Sales and settlements	0			0	174
Transfers into Level 3	0			0	0
Transfers out of Level 3	0			0	0
Balance, end of period	35,062		51,989	35,062	51,989
Movement in total gains (losses) above relating to instruments still held at the reporting date	885		(2,344)	7,582	(12,620)
Corporate
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period	33,511		4,786	23,818	36,866
Realized gains (losses)	20		23	4	(257)
Movement in unrealized gains (losses)	673		(22)	346	173
Purchases and issuances	0		0	8,949	6,878
Sales and settlements	(555)		0	(181)	(10,049)
Transfers into Level 3	0		0	838	0
Transfers out of Level 3	(4,514)		0	(4,639)	(28,824)
Fixed maturities to short term investments classification change				0
Balance, end of period	29,135		4,787	29,135	4,787
Movement in total gains (losses) above relating to instruments still held at the reporting date	674		1	351	(52)
RMBS - Agency
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period	52,588		15,385	32,041	30,255
Realized gains (losses)	4		(81)	(23)	(81)
Movement in unrealized gains (losses)	106		52	58	2
Purchases and issuances	34		33	103	33
Sales and settlements	(4,645)		(206)	(6,233)	(492)
Transfers into Level 3	0		0	15,254	0
Transfers out of Level 3	(14,230)		0	(7,343)	(14,534)
Fixed maturities to short term investments classification change				0
Balance, end of period	33,857		15,183	33,857	15,183
Movement in total gains (losses) above relating to instruments still held at the reporting date	110		(29)	35	(79)
RMBS - Non-Agency
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period	1,596		3,194	0	4,964
Realized gains (losses)	(22)		(328)	0	(329)
Movement in unrealized gains (losses)	(424)		76	0	61
Purchases and issuances	6,821		0	6,821	0
Sales and settlements	(59)		(143)	0	(517)
Transfers into Level 3	0		0	1,018	0
Transfers out of Level 3	(73)		0	0	(1,380)
Fixed maturities to short term investments classification change				0
Balance, end of period	7,839		2,799	7,839	2,799
Movement in total gains (losses) above relating to instruments still held at the reporting date	(447)		(252)	0	(268)
CMBS
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period			4,563	0	1,623
Realized gains (losses)	0		(501)	0	(1,179)
Movement in unrealized gains (losses)	0		564	0	1,078
Purchases and issuances	33,369		0	33,369	3,155
Sales and settlements	0		(232)	0	(283)
Transfers into Level 3	0		0	0	0
Transfers out of Level 3	0		0	0	0
Fixed maturities to short term investments classification change				0
Balance, end of period	33,369		4,394	33,369	4,394
Movement in total gains (losses) above relating to instruments still held at the reporting date	0	0	63	0	(855)
CDO
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period	642,179		727,239	650,851	721,572
Realized gains (losses)	913		(2,662)	(523)	(3,733)
Movement in unrealized gains (losses)	35,886		(46,520)	75,489	(10,104)
Purchases and issuances	0		0	0	2,379
Sales and settlements	(5,347)		(12,215)	(51,024)	(46,158)
Transfers into Level 3	1,162		0	0	1,886
Transfers out of Level 3	0		0	0	0
Fixed maturities to short term investments classification change				0
Balance, end of period	674,793		665,842	674,793	665,842
Movement in total gains (losses) above relating to instruments still held at the reporting date	35,806		(49,432)	71,738	(15,656)
Other asset-backed securities
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period	25,248		17,048	16,552	24,650
Realized gains (losses)	7,017		(238)	6,748	(555)
Movement in unrealized gains (losses)	(6,552)		302	(2,873)	7,162
Purchases and issuances	8,753		0	8,753	0
Sales and settlements	(16,141)		0	(15,867)	(9,114)
Transfers into Level 3	0		0	3,850	0
Transfers out of Level 3	(1,162)		0	0	(5,031)
Fixed maturities to short term investments classification change				0
Balance, end of period	17,163		17,112	17,163	17,112
Movement in total gains (losses) above relating to instruments still held at the reporting date	456		64	3,652	6,287
Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period					3,667
Realized gains (losses)					0
Movement in unrealized gains (losses)					0
Purchases and issuances					0
Sales and settlements					0
Transfers into Level 3					0
Transfers out of Level 3					(3,667)
Balance, end of period			0		0
Movement in total gains (losses) above relating to instruments still held at the reporting date					0
Other investments
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period	117,765		114,540	113,959	133,717
Realized gains (losses)	343		(244)	2,782	11,911
Movement in unrealized gains (losses)	1,686		2,642	6,206	10,134
Purchases and issuances	982		1,667	4,664	9,782
Sales and settlements	(807)		(4,243)	(7,327)	(51,182)
Transfers into Level 3	6		0	6	0
Transfers out of Level 3	0		0	(315)	0
Fixed maturities to short term investments classification change				0
Balance, end of period	119,975		114,362	119,975	114,362
Movement in total gains (losses) above relating to instruments still held at the reporting date	$ 1,334		$ 2,398	$ 2,385	$ 19,735