Fair Value Measurements (Detail) - Additional information about assets and liabilities measured at fair value on a recurring basis (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Corporate [Member]
Balance, beginning of period	$ 23,818	$ 36,866
Realized gains (losses)	(18)	0
Movement in unrealized gains (losses)	(315)	46
Purchases and issuances	9,076	10,629
Sales and settlements	(137)	(1,970)
Transfers into Level 3	881	4,397
Transfers out of Level 3	0	(15,083)
Fixed maturities to short-term investments classification change	0	0
Balance, end of period	33,305	34,885
Movement in total gains (losses) above relating to instruments still held at the reporting date	(332)	51
Balance, beginning of period	33,305	34,885
Residential Mortgage-Backed Securities Agency [Member]
Balance, beginning of period	32,041	30,255
Realized gains (losses)	13	0
Movement in unrealized gains (losses)	(79)	38
Purchases and issuances	36	11,460
Sales and settlements	(2,348)	(1,269)
Transfers into Level 3	10,132	3,944
Transfers out of Level 3	0	(11,441)
Fixed maturities to short-term investments classification change	0	0
Balance, end of period	39,795	32,987
Movement in total gains (losses) above relating to instruments still held at the reporting date	(54)	38
Balance, beginning of period	39,795	32,987
Residential Mortgage-Backed Securities Non-Agency [Member]
Balance, beginning of period	0	4,964
Realized gains (losses)	0	0
Movement in unrealized gains (losses)	0	6
Purchases and issuances	0	0
Sales and settlements	0	(301)
Transfers into Level 3	0	0
Transfers out of Level 3	0	(1,334)
Fixed maturities to short-term investments classification change	0	0
Balance, end of period	0	3,335
Movement in total gains (losses) above relating to instruments still held at the reporting date	0	6
Balance, beginning of period	0	3,335
Commercial mortgage-backed securities [Member]
Balance, beginning of period	0	1,623
Realized gains (losses)	0	(889)
Movement in unrealized gains (losses)	0	1,040
Purchases and issuances	0	0
Sales and settlements	0	(17)
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	1,757
Movement in total gains (losses) above relating to instruments still held at the reporting date	0	151
Balance, beginning of period	0	1,757
Collateralized Debt Obligations [Member]
Balance, beginning of period	650,851	721,572
Realized gains (losses)	(1,649)	(652)
Movement in unrealized gains (losses)	33,116	26,045
Purchases and issuances	0	0
Sales and settlements	(43,621)	(4,414)
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	638,697	742,551
Movement in total gains (losses) above relating to instruments still held at the reporting date	29,202	25,393
Balance, beginning of period	638,697	742,551
Other Asset Backed Securities [Member]
Balance, beginning of period	16,552	24,650
Realized gains (losses)	21	(452)
Movement in unrealized gains (losses)	(163)	2,818
Purchases and issuances	0	0
Sales and settlements	0	(9,650)
Transfers into Level 3	0	0
Transfers out of Level 3	0	(4,995)
Fixed maturities to short-term investments classification change	0	0
Balance, end of period	16,410	12,371
Movement in total gains (losses) above relating to instruments still held at the reporting date	(431)	1,912
Balance, beginning of period	16,410	12,371
Non US Sovereign Government Supranational And Government Related [Member]
Balance, beginning of period	0	3,667
Realized gains (losses)	0	0
Movement in unrealized gains (losses)	0	0
Purchases and issuances	0	0
Sales and settlements	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	(3,667)
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
Balance, beginning of period	0	0
Short-term Investments [Member]
Balance, beginning of period	0	0
Realized gains (losses)	0	0
Movement in unrealized gains (losses)	0	0
Purchases and issuances	0	0
Sales and 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
Balance, beginning of period	0	0
Other Investments [Member]
Balance, beginning of period	113,959	133,717
Realized gains (losses)	1,925	0
Movement in unrealized gains (losses)	899	9,771
Purchases and issuances	1,317	1,864
Sales and settlements	(2,441)	(518)
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	115,659	144,834
Movement in total gains (losses) above relating to instruments still held at the reporting date	(313)	8,707
Balance, beginning of period	115,659	144,834
Derivative Contracts [Member]
Balance, beginning of period	(42,644)	(39,195)
Realized gains (losses)	0	0
Movement in unrealized gains (losses)	2,014	2,449
Purchases and issuances	0	0
Sales and settlements	0	(64)
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	(40,630)	(36,810)
Movement in total gains (losses) above relating to instruments still held at the reporting date	2,014	2,449
Balance, beginning of period	$ (40,630)	$ (36,810)