Fair Value Measurements (Detail) - Additional information about assets and liabilities measured at fair value on a recurring basis (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Corporate [Member]
Balance, beginning of period	$ 34,866	$ 9,493	$ 35,158	$ 10,311
Realized gains (losses)	(247)	117	(279)	(4,438)
Movement in unrealized gains (losses)	354	33	205	480
Purchases and issuances		22,944	6,877	23,275
Sales and settlements	(7,567)	(257)	(8,369)	(1,135)
Transfers into Level 3		12,146		14,951
Transfers out of Level 3	(22,638)	(4,800)	(28,824)	(3,768)
Balance, end of period	4,768	39,676	4,768	39,676
Movement in total gains (losses) above relating to instruments still held at the reporting date	71	316	6	599
Residential Mortgage Backed Securities Agency [Member]
Balance, beginning of period	32,987		30,255	7,894
Movement in unrealized gains (losses)	61		(50)
Purchases and issuances		1,226		1,226
Sales and settlements	(165)		(286)
Transfers into Level 3	11,276
Transfers out of Level 3	(28,774)		(14,534)	(7,894)
Balance, end of period	15,385	1,226	15,385	1,226
Movement in total gains (losses) above relating to instruments still held at the reporting date	61		(50)
Residential Mortgage Backed Securities Non Agency [Member]
Balance, beginning of period	3,335	30,000	4,964	42,190
Realized gains (losses)		(2,681)		(5,674)
Movement in unrealized gains (losses)	(22)	474	(16)	706
Purchases and issuances				9
Sales and settlements	(119)	(1,748)	(374)	(853)
Transfers into Level 3		7,201		8,433
Transfers out of Level 3		(23,238)	(1,380)	(34,803)
Balance, end of period	3,194	10,008	3,194	10,008
Movement in total gains (losses) above relating to instruments still held at the reporting date	(22)	340	(16)	405
Commercial Mortgage Backed Securities [Member]
Balance, beginning of period	1,757	41,655	1,623	2,755
Realized gains (losses)	207	(286)	(678)	(209)
Movement in unrealized gains (losses)	(527)	(60)	514	16
Purchases and issuances	3,155		3,155
Sales and settlements	(29)		(51)	(706)
Transfers out of Level 3		(40,882)		(1,429)
Balance, end of period	4,563	427	4,563	427
Movement in total gains (losses) above relating to instruments still held at the reporting date	(320)	(54)	(152)	(151)
Collateralized Debt Obligations [Member]
Balance, beginning of period	742,284	711,749	721,097	190,663
Realized gains (losses)	(419)	(7,930)	(1,071)	(13,174)
Movement in unrealized gains (losses)	9,644	(13,889)	36,471	16,636
Purchases and issuances	2,379		2,379
Sales and settlements	(29,274)	(467)	(33,523)	(14)
Transfers into Level 3	2,625		1,886	495,734
Transfers out of Level 3		(1,280)		(1,356)
Fixed maturities to short-term investments classification change		(1,416)		(1,722)
Balance, end of period	727,239	686,767	727,239	686,767
Movement in total gains (losses) above relating to instruments still held at the reporting date	8,419	(15,385)	33,777	15,574
Other Asset Backed Securities [Member]
Balance, beginning of period	12,371	52,082	24,650	38,179
Realized gains (losses)	161	(7,742)	(317)	(14,082)
Movement in unrealized gains (losses)	100	5,490	1,698	9,236
Purchases and issuances				4,513
Sales and settlements	(207)	(75)	(9,857)	(76)
Transfers into Level 3				204
Transfers out of Level 3		(27,234)	(5,031)	(15,453)
Fixed maturities to short-term investments classification change	(6,112)		(4,830)
Balance, end of period	6,313	22,521	6,313	22,521
Movement in total gains (losses) above relating to instruments still held at the reporting date	290	(3,221)	1,031	(1,168)
NonUS Sovereign Government Supranational And Government Related [Member]
Balance, beginning of period		3,242	3,667	3,217
Movement in unrealized gains (losses)		(60)		(102)
Transfers into Level 3				67
Transfers out of Level 3			(3,667)
Balance, end of period		3,182		3,182
Movement in total gains (losses) above relating to instruments still held at the reporting date		(60)		(102)
Short-term Investments [Member]
Balance, beginning of period	286	6,566	2,183	6,486
Realized gains (losses)		(1,146)		(4,502)
Movement in unrealized gains (losses)	4,576	(160)	5,098	2,512
Sales and settlements	(221)	(717)	(1,358)	(2,124)
Transfers into Level 3				987
Transfers out of Level 3		(878)
Fixed maturities to short-term investments classification change	6,112	1,416	4,830	1,722
Balance, end of period	10,753	5,081	10,753	5,081
Movement in total gains (losses) above relating to instruments still held at the reporting date	4,588	(58)	5,162	2,193
Other Investments [Member]
Balance, beginning of period	144,834	83,088	133,717	75,584
Realized gains (losses)	12,155	2,565	12,155	2,565
Movement in unrealized gains (losses)	(1,982)	1,567	7,492	5,552
Purchases and issuances	6,254	6,162	8,115	10,908
Sales and settlements	(46,421)	(525)	(46,939)	(1,752)
Transfers out of Level 3	(300)
Balance, end of period	114,540	92,857	114,540	92,857
Movement in total gains (losses) above relating to instruments still held at the reporting date	9,285	1,567	17,337	5,552
Derivative Contracts [Member]
Balance, beginning of period	(36,810)	104,474	(39,195)	100,515
Movement in unrealized gains (losses)	(12,725)	53,027	(10,276)	51,637
Purchases and issuances		5,522		10,985
Sales and settlements	(110)	(112)	(174)	(226)
Balance, end of period	(49,645)	162,911	(49,645)	162,911
Movement in total gains (losses) above relating to instruments still held at the reporting date	$ (12,725)	$ 53,027	$ (10,276)	$ 51,637