Fair Value Measurements - Additional information about assets and liabilities measured at fair value on a recurring basis (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Corporate
Level 3 assets
Balance, beginning of period	$ 30,098	$ 23,818
Realized gains (losses)	(2)	(18)
Movement in unrealized gains (losses)	204	(315)
Purchases and issuances	10,621	9,076
Sales	0	0
Settlements	(106)	(137)
Transfers into Level 3	0	881
Transfers out of Level 3	0	0
Fixed maturities to short-term investments classification change	0	0
Balance, end of period	40,815	33,305
Movement in total gains (losses) above relating to instruments still held at the reporting date	202	(332)
RMBS - Agency
Level 3 assets
Balance, beginning of period	32,005	32,041
Realized gains (losses)	13	13
Movement in unrealized gains (losses)	(18)	(79)
Purchases and issuances	0	36
Sales	(535)	(1,334)
Settlements	(893)	(1,014)
Transfers into Level 3	0	10,132
Transfers out of Level 3	(11,465)	0
Fixed maturities to short-term investments classification change	0	0
Balance, end of period	19,107	39,795
Movement in total gains (losses) above relating to instruments still held at the reporting date	(5)	(54)
RMBS - Non-Agency
Level 3 assets
Balance, beginning of period	116	0
Realized gains (losses)	0	0
Movement in unrealized gains (losses)	0	0
Purchases and issuances	0	0
Sales	0	0
Settlements	(13)	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Fixed maturities to short-term investments classification change	0	0
Balance, end of period	103	0
Movement in total gains (losses) above relating to instruments still held at the reporting date	0	0
CMBS
Level 3 assets
Balance, beginning of period	25,347	0
Realized gains (losses)	0	0
Movement in unrealized gains (losses)	1	0
Purchases and issuances	0	0
Sales	0	0
Settlements	(336)	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Fixed maturities to short-term investments classification change	0	0
Balance, end of period	25,012	0
Movement in total gains (losses) above relating to instruments still held at the reporting date	1	0
CDO
Level 3 assets
Balance, beginning of period	701,736	650,851
Realized gains (losses)	356	(1,649)
Movement in unrealized gains (losses)	21,255	33,116
Purchases and issuances	0	0
Sales	0	(37,181)
Settlements	(20,443)	(6,440)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Fixed maturities to short-term investments classification change	0	0
Balance, end of period	702,904	638,697
Movement in total gains (losses) above relating to instruments still held at the reporting date	20,892	29,202
Other asset-backed securities
Level 3 assets
Balance, beginning of period	18,128	16,552
Realized gains (losses)	51	21
Movement in unrealized gains (losses)	1,475	(163)
Purchases and issuances	21,686	0
Sales	0	0
Settlements	(5,453)	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Fixed maturities to short-term investments classification change	0	0
Balance, end of period	35,887	16,410
Movement in total gains (losses) above relating to instruments still held at the reporting date	7	(431)
Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Level 3 assets
Balance, beginning of period	0	0
Realized gains (losses)	0	0
Movement in unrealized gains (losses)	0	0
Purchases and issuances	0	0
Sales	0	0
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	0
Balance, end of period	0	0
Movement in total gains (losses) above relating to instruments still held at the reporting date	0	0
Short-term investments
Level 3 assets
Balance, beginning of period	0	0
Realized gains (losses)	0	0
Movement in unrealized gains (losses)	0	0
Purchases and issuances	2,017	0
Sales	0	0
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	0
Balance, end of period	2,017	0
Movement in total gains (losses) above relating to instruments still held at the reporting date	0	0
Other investments
Level 3 assets
Balance, beginning of period	115,272	113,959
Realized gains (losses)	2,915	1,925
Movement in unrealized gains (losses)	(1,902)	899
Purchases and issuances	1,905	1,317
Sales	0	0
Settlements	(4,868)	(2,441)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Fixed maturities to short-term investments classification change	0	0
Balance, end of period	113,322	115,659
Movement in total gains (losses) above relating to instruments still held at the reporting date	1,013	(313)
Derivative contracts - net
Level 3 liabilities
Balance, beginning of period	(36,247)	(42,644)
Realized gains (losses)	0	0
Movement in unrealized gains (losses)	3,697	2,014
Purchases and issuances	0	0
Sales	0	0
Settlements	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance, end of period	(32,550)	(40,630)
Movement in total gains (losses) above relating to instruments still held at the reporting date	$ 3,697	$ 2,014