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		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Derivative Financial Instruments Liabilities Member
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period	$ 40,630	$ 36,810	$ 42,644	$ 39,195
Realized gains (losses)			0
Movement in unrealized gains (losses)	(4,683)	12,725	(6,697)	10,276
Purchases and issuances		(110)	0
Sales and settlements			0	174
Transfers into Level 3			0
Transfers out of Level 3			0
Balance, end of period	35,947	49,645	35,947	49,645
Movement in total gains (losses) above relating to instruments still held at the reporting date	4,683	(12,725)	6,697	(10,276)
Corporate
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period	33,305	34,885	23,818	36,866
Realized gains (losses)	82	(247)	(42)	(279)
Movement in unrealized gains (losses)	94	353	1,255	195
Purchases and issuances	0	0	8,734	6,877
Sales and settlements	(213)	(7,567)	(1,608)	(10,049)
Transfers into Level 3	536	0	1,354	0
Transfers out of Level 3	(293)	(22,638)	0	(28,824)
Fixed maturities to short term investments classification change			0
Balance, end of period	33,511	4,786	33,511	4,786
Movement in total gains (losses) above relating to instruments still held at the reporting date	94	71	1,255	6
RMBS - Agency
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period	39,795	32,987	32,041	30,255
Realized gains (losses)	(30)	0	(96)	0
Movement in unrealized gains (losses)	195	61	(95)	(50)
Purchases and issuances	35	0	70	0
Sales and settlements	(4,345)	(165)	(3,211)	(286)
Transfers into Level 3	16,938	11,276	23,879	0
Transfers out of Level 3		(28,774)	0	(14,534)
Fixed maturities to short term investments classification change			0
Balance, end of period	52,588	15,385	52,588	15,385
Movement in total gains (losses) above relating to instruments still held at the reporting date	197	61	(95)	(50)
RMBS - Non-Agency
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period		3,335	0	4,964
Realized gains (losses)		0	0	0
Movement in unrealized gains (losses)		(22)	0	(16)
Purchases and issuances		0	0	0
Sales and settlements		(119)	0	(374)
Transfers into Level 3	1,596	0	1,596	0
Transfers out of Level 3		0	0	(1,380)
Fixed maturities to short term investments classification change			0
Balance, end of period	1,596	3,194	1,596	3,194
Movement in total gains (losses) above relating to instruments still held at the reporting date		(22)	0	(16)
CMBS
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period		1,757	0	1,623
Realized gains (losses)		207	0	(678)
Movement in unrealized gains (losses)		(527)	0	514
Purchases and issuances		3,155	0	3,155
Sales and settlements		(29)	0	(51)
Transfers into Level 3		0	0	0
Transfers out of Level 3		0	0	0
Fixed maturities to short term investments classification change			0
Balance, end of period	0	4,563	0	4,563
Movement in total gains (losses) above relating to instruments still held at the reporting date		(320)	0	(152)
CDO
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period	638,697	742,551	650,851	721,572
Realized gains (losses)	57	(419)	(1,612)	(1,071)
Movement in unrealized gains (losses)	6,186	9,633	39,379	36,417
Purchases and issuances	0	2,379	0	2,379
Sales and settlements	(1,488)	(29,530)	(44,835)	(33,944)
Transfers into Level 3	0	2,625	0	1,886
Transfers out of Level 3	(1,273)	0	(1,604)	0
Fixed maturities to short term investments classification change			0
Balance, end of period	642,179	727,239	642,179	727,239
Movement in total gains (losses) above relating to instruments still held at the reporting date	6,186	8,419	35,379	33,777
Other asset-backed securities
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period	16,410	12,371	16,552	24,650
Realized gains (losses)	(135)	161	(94)	(317)
Movement in unrealized gains (losses)	4,145	4,688	3,905	6,860
Purchases and issuances	0	0	0	0
Sales and settlements	(844)	(172)	(1,118)	(9,114)
Transfers into Level 3	5,672	0	6,003	0
Transfers out of Level 3		0	0	(5,031)
Fixed maturities to short term investments classification change			0
Balance, end of period	25,248	17,048	25,248	17,048
Movement in total gains (losses) above relating to instruments still held at the reporting date	3,918	4,878	3,681	6,193
Non U.S. sovereign government, supranational, and government related
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period		0	0	3,667
Realized gains (losses)		0	0	0
Movement in unrealized gains (losses)		0	0	0
Purchases and issuances		0	0	0
Sales and settlements		0	0	0
Transfers into Level 3		0	0	0
Transfers out of Level 3		0	0	(3,667)
Fixed maturities to short term investments classification change			0
Balance, end of period	0	0	0	0
Movement in total gains (losses) above relating to instruments still held at the reporting date		0	0	0
Short-term investments
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period			0	0
Realized gains (losses)			0	0
Movement in unrealized gains (losses)			0	0
Purchases and issuances			0	0
Sales and settlements			0	0
Transfers into Level 3			0	0
Transfers out of Level 3			0	0
Fixed maturities to short term investments classification change			0
Balance, end of period	0	0	0	0
Movement in total gains (losses) above relating to instruments still held at the reporting date			0	0
Other investments
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period	115,659	144,834	113,959	133,717
Realized gains (losses)	510	12,155	2,438	12,155
Movement in unrealized gains (losses)	3,616	(1,982)	4,521	7,492
Purchases and issuances	2,365	6,254	3,682	8,115
Sales and settlements	(4,080)	(46,421)	(6,520)	(46,939)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	(305)	(300)	(315)	0
Fixed maturities to short term investments classification change			0
Balance, end of period	117,765	114,540	117,765	114,540
Movement in total gains (losses) above relating to instruments still held at the reporting date	$ 1,326	$ 9,285	$ 1,004	$ 17,337