Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011		Dec. 31, 2011
Segment Reporting Information [Line Items]
Gross written premiums	$ 322,987		$ 278,714		$ 666,136		$ 574,997
Net written premiums	190,252		183,363		433,297		376,439
Net earned premiums	196,017		173,777		379,136		326,255
Net losses and loss adjustment expenses	(123,407)		(113,863)		(241,392)		(230,651)
Commission expenses	(29,503)		(28,030)		(58,953)		(54,230)
Other operating expenses	(39,819)		(35,777)		(76,126)		(72,352)
Other income (expense)	387		573		1,298		1,564
Underwriting profit (loss)	3,675		(3,320)		3,963		(29,414)
Net investment income	15,777		17,429		27,035		34,813
Net realized gains (losses)	3,721		2,474		5,409		844
Interest expense	(2,049)		(2,047)		(4,098)		(4,093)
Income (loss) before income taxes	21,124		14,536		32,309		2,150
Income tax expense (benefit)	6,225		5,032		9,506		539
Net income (loss)	14,899		9,504		22,803		1,611
Identifiable assets	3,939,066		3,655,909		3,939,066		3,655,909		3,670,007
Losses and loss adjustment expenses ratio	63.00%		65.50%		63.70%		70.70%
Commission expense ratio	15.10%		16.10%		15.50%		16.60%
Other operating expense ratio	20.00%	[1]	20.30%	[1]	19.80%	[1]	21.70%	[1]
Combined ratio	98.10%		101.90%		99.00%		109.00%
Insurance Companies
Segment Reporting Information [Line Items]
Gross written premiums	214,064		186,767		462,402		393,543
Net written premiums	134,065		123,204		315,315		253,944
Net earned premiums	141,577		114,987		273,125		213,807
Net losses and loss adjustment expenses	(100,003)		(77,330)		(191,180)		(152,127)
Commission expenses	(21,117)		(16,402)		(40,418)		(28,742)
Other operating expenses	(28,914)		(26,516)		(54,259)		(53,315)
Other income (expense)	879		626		2,521		2,317
Underwriting profit (loss)	(7,578)		(4,635)		(10,211)		(18,060)
Net investment income	13,286		14,989		22,221		29,972
Net realized gains (losses)	2,325		3,100		4,200		2,855
Income (loss) before income taxes	8,033		13,454		16,210		14,767
Income tax expense (benefit)	2,101		4,617		3,980		4,845
Net income (loss)	5,932		8,837		12,230		9,922
Identifiable assets	2,933,334		2,716,634		2,933,334		2,716,634
Losses and loss adjustment expenses ratio	70.60%		67.30%		70.00%		71.20%
Commission expense ratio	14.90%		14.30%		14.80%		13.40%
Other operating expense ratio	19.90%	[1]	22.40%	[1]	18.90%	[1]	23.80%	[1]
Combined ratio	105.40%		104.00%		103.70%		108.40%
Lloyd's Operations
Segment Reporting Information [Line Items]
Gross written premiums	108,923		91,947		203,734		181,454
Net written premiums	56,187		60,159		117,982		122,495
Net earned premiums	54,440		58,790		106,011		112,448
Net losses and loss adjustment expenses	(23,404)		(36,533)		(50,212)		(78,524)
Commission expenses	(8,938)		(12,042)		(19,824)		(26,449)
Other operating expenses	(10,905)		(9,261)		(21,867)		(19,037)
Other income (expense)	60		361		66		208
Underwriting profit (loss)	11,253		1,315		14,174		(11,354)
Net investment income	2,454		2,320		4,737		4,575
Net realized gains (losses)	1,396		(798)		1,209		(2,183)
Income (loss) before income taxes	15,103		2,837		20,120		(8,962)
Income tax expense (benefit)	5,207		1,029		6,933		(3,027)
Net income (loss)	9,896		1,808		13,187		(5,935)
Identifiable assets	967,862		898,895		967,862		898,895
Losses and loss adjustment expenses ratio	43.00%		62.10%		47.40%		69.80%
Commission expense ratio	16.40%		20.50%		18.70%		23.50%
Other operating expense ratio	19.90%	[1]	15.20%	[1]	20.50%	[1]	16.80%	[1]
Combined ratio	79.30%		97.80%		86.60%		110.10%
Corporate
Segment Reporting Information [Line Items]
Commission expenses	552	[2]	414	[2]	1,289	[2]	961	[2]
Other income (expense)	(552)	[2]	(414)	[2]	(1,289)	[2]	(961)	[2]
Net investment income	37	[2]	120	[2]	77	[2]	266	[2]
Net realized gains (losses)			172	[2]			172	[2]
Interest expense	(2,049)	[2]	(2,047)	[2]	(4,098)	[2]	(4,093)	[2]
Income (loss) before income taxes	(2,012)	[2]	(1,755)	[2]	(4,021)	[2]	(3,655)	[2]
Income tax expense (benefit)	(1,083)	[2]	(614)	[2]	(1,407)	[2]	(1,279)	[2]
Net income (loss)	(929)	[2]	(1,141)	[2]	(2,614)	[2]	(2,376)	[2]
Identifiable assets	$ 37,870	[2]	$ 40,380	[2]	$ 37,870	[2]	$ 40,380	[2]

[1] Includes Other operating expenses and Other income.

[2] Includes Corporate segment intercompany eliminations.