Fair Value Measurements (Details) - Additional information about assets and liabilities measured at fair value on a recurring basis (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Derivative Contracts - Net
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period	$ 42,644	$ 39,195
Realized gains (losses)	0	0
Movement in unrealized gains (losses)	(6,397)	3,173
Purchases and issuances	0	0
Sales and settlements	0	276
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance, end of period	36,247	42,644
Movement in total gains (losses) above relating to instruments still held at the reporting date	6,397	(3,173)
Corporate
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period	23,818	36,866
Realized gains (losses)	4	(276)
Movement in unrealized gains (losses)	100	92
Purchases and issuances	10,278	14,840
Sales and settlements	(282)	(10,049)
Transfers into Level 3	819	2,105
Transfers out of Level 3	(4,639)	(19,760)
Fixed maturities to short term investments classification change	0
Balance, end of period	30,098	23,818
Movement in total gains (losses) above relating to instruments still held at the reporting date	122	92
RMBS - Agency
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period	32,041	30,255
Realized gains (losses)	(15)	(11)
Movement in unrealized gains (losses)	27	(145)
Purchases and issuances	0	6,176
Sales and settlements	(6,307)	(4,186)
Transfers into Level 3	21,249	2,655
Transfers out of Level 3	(14,990)	(2,703)
Fixed maturities to short term investments classification change	0
Balance, end of period	32,005	32,041
Movement in total gains (losses) above relating to instruments still held at the reporting date	10	(156)
RMBS - Non-Agency
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period	0	4,964
Realized gains (losses)	0
Movement in unrealized gains (losses)	0
Purchases and issuances	0	0
Sales and settlements	0	0
Transfers into Level 3	116	0
Transfers out of Level 3	0	(4,964)
Fixed maturities to short term investments classification change	0
Balance, end of period	116	0
Movement in total gains (losses) above relating to instruments still held at the reporting date	0	0
CMBS
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period	0	1,623
Realized gains (losses)	0
Movement in unrealized gains (losses)	20
Purchases and issuances	25,546	0
Sales and settlements	(219)	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	(1,623)
Fixed maturities to short term investments classification change	0
Balance, end of period	25,347	0
Movement in total gains (losses) above relating to instruments still held at the reporting date	20	0
CDO
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period	650,851	721,572
Realized gains (losses)	(894)	(3,458)
Movement in unrealized gains (losses)	109,282	2,404
Purchases and issuances	0	2,379
Sales and settlements	(57,032)	(68,165)
Transfers into Level 3	0	1,886
Transfers out of Level 3	(471)	(5,767)
Fixed maturities to short term investments classification change	0
Balance, end of period	701,736	650,851
Movement in total gains (losses) above relating to instruments still held at the reporting date	104,474	(6,431)
Other asset-backed securities
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance, beginning of period	16,552	24,650
Realized gains (losses)	6,768	(849)
Movement in unrealized gains (losses)	(2,784)	6,896
Purchases and issuances	8,753	0
Sales and settlements	(16,094)	(9,114)
Transfers into Level 3	4,933	0
Transfers out of Level 3	0	(5,031)
Fixed maturities to short term investments classification change	0
Balance, end of period	18,128	16,552
Movement in total gains (losses) above relating to instruments still held at the reporting date	(154)	12,334
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
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
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	113,959	133,717
Realized gains (losses)	5,083	11,592
Movement in unrealized gains (losses)	7,802	14,108
Purchases and issuances	7,375	12,177
Sales and settlements	(18,637)	(57,635)
Transfers into Level 3	5	0
Transfers out of Level 3	(315)	0
Fixed maturities to short term investments classification change	0
Balance, end of period	115,272	113,959
Movement in total gains (losses) above relating to instruments still held at the reporting date	$ 5,038	$ 23,391