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		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Corporate
Level 3 assets
Balance, beginning of period	$ 4,382	$ 40,815	$ 31,573	$ 30,098
Realized gains (losses)	35	(69)	155	(71)
Movement in unrealized gains (losses)	(93)	426	(96)	630
Purchases and issuances	8	0	1,443	10,621
Sales	0	0	0	0
Settlements	(399)	(7,139)	(5,513)	(7,245)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	(1,860)	(23,629)	(1,860)
Fixed maturities to short-term investments classification change	0	0	0	0
Balance, end of period	3,933	32,173	3,933	32,173
Movement in total gains (losses) above relating to instruments still held at the reporting date	(58)	424	90	627
RMBS - Agency
Level 3 assets
Balance, beginning of period	8,928	19,107	10,473	32,005
Realized gains (losses)	6	(2)	6	11
Movement in unrealized gains (losses)	(13)	(19)	(13)	(37)
Purchases and issuances	0	0	0	0
Sales	0	0	0	(535)
Settlements	(2,025)	(872)	(2,598)	(1,765)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	(972)	(11,465)
Fixed maturities to short-term investments classification change	0	0	0	0
Balance, end of period	6,896	18,214	6,896	18,214
Movement in total gains (losses) above relating to instruments still held at the reporting date	(6)	(22)	(6)	(40)
RMBS - Non-Agency
Level 3 assets
Balance, beginning of period	11	103	9	116
Realized gains (losses)	0	0	0	0
Movement in unrealized gains (losses)	0	0	2	0
Purchases and issuances	0	3,326	0	3,326
Sales	0	0	0	0
Settlements	0	(18)	0	(31)
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	11	3,411	11	3,411
Movement in total gains (losses) above relating to instruments still held at the reporting date	0	(1)	2	(1)
CMBS
Level 3 assets
Balance, beginning of period	5,926	25,012	12,533	25,347
Realized gains (losses)	2	0	3	0
Movement in unrealized gains (losses)	(1)	(19)	(3)	(18)
Purchases and issuances	1,376	0	1,376	0
Sales	0	0	0	0
Settlements	(5,358)	(570)	(11,964)	(906)
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	1,945	24,423	1,945	24,423
Movement in total gains (losses) above relating to instruments still held at the reporting date	0	(19)	0	(18)
CDO
Level 3 assets
Balance, beginning of period	718,827	702,904	710,253	701,736
Realized gains (losses)	875	(55)	2,456	301
Movement in unrealized gains (losses)	8,331	14,491	12,013	35,746
Purchases and issuances	75,201	5,362	103,015	5,362
Sales	(30,892)	0	(40,824)	0
Settlements	(39,518)	(16,689)	(54,089)	(37,132)
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	732,824	706,013	732,824	706,013
Movement in total gains (losses) above relating to instruments still held at the reporting date	8,247	13,804	13,216	34,278
Other asset-backed securities
Level 3 assets
Balance, beginning of period	10,673	35,887	11,877	18,128
Realized gains (losses)	(5)	(13)	(20)	38
Movement in unrealized gains (losses)	144	(512)	206	963
Purchases and issuances	3,000	0	3,000	21,686
Sales	0	0	0	0
Settlements	(2,108)	(1,317)	(3,359)	(6,770)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	(6,506)	0	(6,506)
Fixed maturities to short-term investments classification change	0	0	0	0
Balance, end of period	11,704	27,539	11,704	27,539
Movement in total gains (losses) above relating to instruments still held at the reporting date	140	(525)	186	(516)
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	0	2,017	2,015	0
Realized gains (losses)	0	(10)	0	(10)
Movement in unrealized gains (losses)	0	(8)	(15)	(8)
Purchases and issuances	0	0	0	2,017
Sales	0	0	0	0
Settlements	0	0	(2,000)	0
Transfers into Level 3	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	1,999	0	1,999
Movement in total gains (losses) above relating to instruments still held at the reporting date	0	(18)	0	(18)
Other investments
Level 3 assets
Balance, beginning of period	116,418	113,322	113,472	115,272
Realized gains (losses)	5,127	2,872	8,691	5,787
Movement in unrealized gains (losses)	(1,779)	932	(282)	(970)
Purchases and issuances	9,997	1,002	21,086	2,907
Sales	0	0	0	0
Settlements	(5,288)	(14,284)	(18,492)	(19,152)
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	124,475	103,844	124,475	103,844
Movement in total gains (losses) above relating to instruments still held at the reporting date	3,348	3,812	8,409	4,808
Derivative contracts - net
Level 3 liabilities
Balance, beginning of period	(32,496)	(32,550)	(29,110)	(36,247)
Realized gains (losses)	0	0	0	0
Movement in unrealized gains (losses)	13,586	3,176	10,200	6,873
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	(18,910)	(29,374)	(18,910)	(29,374)
Movement in total gains (losses) above relating to instruments still held at the reporting date	$ 13,586	$ 3,176	$ 10,200	$ 6,873