Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010		Dec. 31, 2010
Segment Reporting Information [Line Items]
Gross written premiums	$ 278,714		$ 253,568		$ 574,997		$ 523,713
Net written premiums	183,363		165,005		376,439		354,322
Net earned premiums	173,777		161,471		326,255		325,540
Net losses and loss adjustment expenses	(113,863)		(99,863)		(230,651)		(203,670)
Commission expenses	(28,030)		(25,677)		(54,230)		(50,993)
Other operating expenses	(35,777)		(34,524)		(72,352)		(69,120)
Other income (expense)	573		(888)		1,564		192
Underwriting profit (loss)	(3,320)		519		(29,414)		1,949
Net investment income	17,429		17,853		34,813		35,825
Net realized gains (losses)	2,474		10,865		844		16,897
Interest expense	(2,047)		(2,044)		(4,093)		(4,088)
Income (loss) before income taxes	14,536		27,193		2,150		50,583
Income tax expense (benefit)	5,032		8,223		539		14,568
Net income (loss)	9,504		18,970		1,611		36,015
Identifiable assets	3,655,909	[1]	3,480,255	[1]	3,655,909	[1]	3,480,255	[1]	3,531,459
Losses and loss adjustment expenses ratio	65.50%		61.80%		70.70%		62.60%
Commission expense ratio	16.10%		15.90%		16.60%		15.70%
Other operating expense ratio	20.30%	[2]	22.00%	[2]	21.70%	[2]	21.10%	[2]
Combined ratio	101.90%		99.70%		109.00%		99.40%
Insurance Companies
Segment Reporting Information [Line Items]
Gross written premiums	186,767		170,641		393,543		348,479
Net written premiums	123,204		111,401		253,944		232,741
Net earned premiums	114,987		110,425		213,807		221,636
Net losses and loss adjustment expenses	(77,330)		(64,862)		(152,127)		(133,265)
Commission expenses	(16,402)		(14,615)		(28,742)		(28,977)
Other operating expenses	(26,516)		(25,907)		(53,315)		(53,260)
Other income (expense)	626		(114)		2,317		(1,091)
Underwriting profit (loss)	(4,635)		4,927		(18,060)		5,043
Net investment income	14,989		15,556		29,972		31,304
Net realized gains (losses)	3,100		10,729		2,855		15,934
Income (loss) before income taxes	13,454		31,212		14,767		52,281
Income tax expense (benefit)	4,617		9,654		4,845		15,117
Net income (loss)	8,837		21,558		9,922		37,164
Identifiable assets	2,681,023	[1]	2,564,004	[1]	2,681,023	[1]	2,564,004	[1]
Losses and loss adjustment expenses ratio	67.30%		58.70%		71.20%		60.10%
Commission expense ratio	14.30%		13.20%		13.40%		13.10%
Other operating expense ratio	22.40%	[2]	23.60%	[2]	23.80%	[2]	24.50%	[2]
Combined ratio	104.00%		95.50%		108.40%		97.70%
Lloyd's Operations
Segment Reporting Information [Line Items]
Gross written premiums	91,947		82,927		181,454		175,234
Net written premiums	60,159		53,604		122,495		121,581
Net earned premiums	58,790		51,046		112,448		103,904
Net losses and loss adjustment expenses	(36,533)		(35,001)		(78,524)		(70,405)
Commission expenses	(12,042)		(11,402)		(26,449)		(22,368)
Other operating expenses	(9,261)		(8,617)		(19,037)		(15,860)
Other income (expense)	361		(434)		208		1,635
Underwriting profit (loss)	1,315		(4,408)		(11,354)		(3,094)
Net investment income	2,320		2,128		4,575		4,197
Net realized gains (losses)	(798)		19		(2,183)		732
Income (loss) before income taxes	2,837		(2,261)		(8,962)		1,835
Income tax expense (benefit)	1,029		(815)		(3,027)		688
Net income (loss)	1,808		(1,446)		(5,935)		1,147
Identifiable assets	908,361	[1]	829,008	[1]	908,361	[1]	829,008	[1]
Losses and loss adjustment expenses ratio	62.10%		68.60%		69.80%		67.80%
Commission expense ratio	20.50%		22.30%		23.50%		21.50%
Other operating expense ratio	15.20%	[2]	17.70%	[2]	16.80%	[2]	13.70%	[2]
Combined ratio	97.80%		108.60%		110.10%		103.00%
Corporate
Segment Reporting Information [Line Items]
Commission expenses	414	[3]	340	[3]	961	[3]	352	[3]
Other income (expense)	(414)	[3]	(340)	[3]	(961)	[3]	(352)	[3]
Net investment income	120	[3]	169	[3]	266	[3]	324	[3]
Net realized gains (losses)	172	[3]	117	[3]	172	[3]	231	[3]
Interest expense	(2,047)	[3]	(2,044)	[3]	(4,093)	[3]	(4,088)	[3]
Income (loss) before income taxes	(1,755)	[3]	(1,758)	[3]	(3,655)	[3]	(3,533)	[3]
Income tax expense (benefit)	(614)	[3]	(616)	[3]	(1,279)	[3]	(1,237)	[3]
Net income (loss)	(1,141)	[3]	(1,142)	[3]	(2,376)	[3]	(2,296)	[3]
Identifiable assets	$ 74,326	[1],[3]	$ 70,742	[1],[3]	$ 74,326	[1],[3]	$ 70,742	[1],[3]

[1] Includes inter-segment transactions causing the row not to cross foot.

[2] Includes Other operating expenses and Other income.

[3] Includes Corporate segment intercompany eliminations.