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, 2014	Mar. 31, 2013
Corporate
Level 3 assets
Balance, beginning of period	$ 31,573	$ 30,098
Realized gains (losses)	120	(2)
Movement in unrealized gains (losses)	(2)	204
Purchases and issuances	1,434	10,621
Sales	0	0
Settlements	(5,114)	(106)
Transfers into Level 3	0	0
Transfers out of Level 3	(23,629)	0
Fixed maturities to short-term investments classification change	0	0
Balance, end of period	4,382	40,815
Movement in total gains (losses) above relating to instruments still held at the reporting date	148	202
RMBS - Agency
Level 3 assets
Balance, beginning of period	10,473	32,005
Realized gains (losses)	(1)	13
Movement in unrealized gains (losses)	1	(18)
Purchases and issuances	0	0
Sales	0	(535)
Settlements	(573)	(893)
Transfers into Level 3	0	0
Transfers out of Level 3	(972)	(11,465)
Fixed maturities to short-term investments classification change	0	0
Balance, end of period	8,928	19,107
Movement in total gains (losses) above relating to instruments still held at the reporting date	0	(5)
RMBS - Non-Agency
Level 3 assets
Balance, beginning of period	9	116
Realized gains (losses)	0	0
Movement in unrealized gains (losses)	2	0
Purchases and issuances	0	0
Sales	0	0
Settlements	0	(13)
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	11	103
Movement in total gains (losses) above relating to instruments still held at the reporting date	2	0
CMBS
Level 3 assets
Balance, beginning of period	12,533	25,347
Realized gains (losses)	2	0
Movement in unrealized gains (losses)	(3)	1
Purchases and issuances	0	0
Sales	0	0
Settlements	(6,606)	(336)
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	5,926	25,012
Movement in total gains (losses) above relating to instruments still held at the reporting date	0	1
CDO
Level 3 assets
Balance, beginning of period	710,253	701,736
Realized gains (losses)	1,581	356
Movement in unrealized gains (losses)	3,681	21,255
Purchases and issuances	27,814	0
Sales	(9,931)	0
Settlements	(14,571)	(20,443)
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	718,827	702,904
Movement in total gains (losses) above relating to instruments still held at the reporting date	4,969	20,892
Other asset-backed securities
Level 3 assets
Balance, beginning of period	11,877	18,128
Realized gains (losses)	(15)	51
Movement in unrealized gains (losses)	62	1,475
Purchases and issuances	0	21,686
Sales	0	0
Settlements	(1,251)	(5,453)
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	10,673	35,887
Movement in total gains (losses) above relating to instruments still held at the reporting date	46	7
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	2,015	0
Realized gains (losses)	0	0
Movement in unrealized gains (losses)	(15)	0
Purchases and issuances	0	2,017
Sales	0	0
Settlements	(2,000)	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	2,017
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	113,472	115,272
Realized gains (losses)	3,566	2,915
Movement in unrealized gains (losses)	1,494	(1,902)
Purchases and issuances	11,089	1,905
Sales	0	0
Settlements	(13,203)	(4,868)
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	116,418	113,322
Movement in total gains (losses) above relating to instruments still held at the reporting date	5,061	1,013
Derivative contracts - net
Level 3 liabilities
Balance, beginning of period	(29,110)	(36,247)
Realized gains (losses)	0	0
Movement in unrealized gains (losses)	(3,386)	3,697
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	(32,496)	(32,550)
Movement in total gains (losses) above relating to instruments still held at the reporting date	$ (3,386)	$ 3,697