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		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Corporate
Level 3 assets
Balance, beginning of period	$ 32,173	$ 33,511	$ 30,098	$ 23,818
Realized gains (losses)	32	20	(39)	4
Movement in unrealized gains (losses)	352	673	982	346
Purchases and issuances	70	0	10,691	8,949
Sales	0	(555)	0	(555)
Settlements	0	0	(7,245)	(181)
Transfers into Level 3	0	0	0	1,393
Transfers out of Level 3	0	(4,514)	(1,860)	(4,639)
Fixed maturities to short-term investments classification change	0	0	0	0
Balance, end of period	32,627	29,135	32,627	29,135
Movement in total gains (losses) above relating to instruments still held at the reporting date	385	674	1,012	351
RMBS - Agency
Level 3 assets
Balance, beginning of period	18,214	52,588	32,005	32,041
Realized gains (losses)	(1)	4	10	(23)
Movement in unrealized gains (losses)	(11)	106	(48)	58
Purchases and issuances	0	34	0	103
Sales	0	0	(535)	0
Settlements	(913)	(4,645)	(2,678)	(6,233)
Transfers into Level 3	0	0	0	22,141
Transfers out of Level 3	0	(14,230)	(11,465)	(14,230)
Fixed maturities to short-term investments classification change	0	0	0	0
Balance, end of period	17,289	33,857	17,289	33,857
Movement in total gains (losses) above relating to instruments still held at the reporting date	(12)	110	(52)	35
RMBS - Non-Agency
Level 3 assets
Balance, beginning of period	3,411	1,596	116	0
Realized gains (losses)	(1)	(22)	(1)	0
Movement in unrealized gains (losses)	(1)	(424)	(1)	0
Purchases and issuances	0	6,821	3,326	6,821
Sales	0	0	0	0
Settlements	(74)	(59)	(105)	0
Transfers into Level 3	0	0	0	1,091
Transfers out of Level 3	(3,326)	(73)	(3,326)	(73)
Fixed maturities to short-term investments classification change	0	0	0	0
Balance, end of period	9	7,839	9	7,839
Movement in total gains (losses) above relating to instruments still held at the reporting date	0	(447)	(1)	0
CMBS
Level 3 assets
Balance, beginning of period	24,423	0	25,347	0
Realized gains (losses)	1	0	1	0
Movement in unrealized gains (losses)	(58)	0	(76)	0
Purchases and issuances	0	33,369	0	33,369
Sales	0	0	0	0
Settlements	(4,406)	0	(5,312)	0
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	0	0	0
Balance, end of period	19,960	33,369	19,960	33,369
Movement in total gains (losses) above relating to instruments still held at the reporting date	(57)	0	(75)	0
CDO
Level 3 assets
Balance, beginning of period	706,013	642,179	701,736	650,851
Realized gains (losses)	584	913	885	(523)
Movement in unrealized gains (losses)	(3,405)	35,886	32,341	75,489
Purchases and issuances	70,552	0	75,914	0
Sales	0	0	0	(32,495)
Settlements	(69,193)	(5,347)	(106,325)	(18,529)
Transfers into Level 3	0	1,162	0	1,162
Transfers out of Level 3	0	0	0	(1,162)
Fixed maturities to short-term investments classification change	0	0	0	0
Balance, end of period	704,551	674,793	704,551	674,793
Movement in total gains (losses) above relating to instruments still held at the reporting date	(3,681)	35,806	30,597	71,738
Other asset-backed securities
Level 3 assets
Balance, beginning of period	27,539	25,248	18,128	16,552
Realized gains (losses)	810	7,017	848	6,748
Movement in unrealized gains (losses)	722	(6,552)	1,685	(2,873)
Purchases and issuances	0	8,753	21,686	8,753
Sales	(15,711)	0	(15,711)	0
Settlements	(906)	(16,141)	(7,676)	(15,867)
Transfers into Level 3	0	0	0	5,012
Transfers out of Level 3	0	(1,162)	(6,506)	(1,162)
Fixed maturities to short-term investments classification change	0	0	0	0
Balance, end of period	12,454	17,163	12,454	17,163
Movement in total gains (losses) above relating to instruments still held at the reporting date	(4)	456	(520)	3,652
Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Level 3 assets
Balance, beginning of period	0	0	0	0
Realized gains (losses)	0	0	0	0
Movement in unrealized gains (losses)	0	0	0	0
Purchases and issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
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	0	0	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	0
Short-term investments
Level 3 assets
Balance, beginning of period	1,999	0	0	0
Realized gains (losses)	(5)	0	(15)	0
Movement in unrealized gains (losses)	64	0	56	0
Purchases and issuances	0	0	2,017	0
Sales	0	0	0	0
Settlements	0	0	0	0
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	0	0	0
Balance, end of period	2,058	0	2,058	0
Movement in total gains (losses) above relating to instruments still held at the reporting date	59	0	41	0
Other investments
Level 3 assets
Balance, beginning of period	103,844	117,765	115,272	113,959
Realized gains (losses)	1,082	343	6,869	2,782
Movement in unrealized gains (losses)	1,974	1,686	1,004	6,206
Purchases and issuances	2,333	982	5,240	4,664
Sales	0	0	0	(7,327)
Settlements	(5,377)	(807)	(24,529)	0
Transfers into Level 3	0	6	0	6
Transfers out of Level 3	0	0	0	(315)
Fixed maturities to short-term investments classification change	0	0	0	0
Balance, end of period	103,856	119,975	103,856	119,975
Movement in total gains (losses) above relating to instruments still held at the reporting date	3,057	1,334	7,865	2,385
Derivative contracts - net
Level 3 liabilities
Balance, beginning of period	(29,374)	(35,947)	(36,247)	(42,644)
Realized gains (losses)	0	0	0	0
Movement in unrealized gains (losses)	(4,015)	885	2,858	7,582
Purchases and issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Balance, end of period	(33,389)	(35,062)	(33,389)	(35,062)
Movement in total gains (losses) above relating to instruments still held at the reporting date	$ (4,015)	$ 885	$ 2,858	$ 7,582