Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011		Dec. 31, 2011
Segment Reporting Information [Line Items]
Gross written premiums	$ 298,742		$ 255,318		$ 964,878		$ 830,315
Net written premiums	188,046		175,357		621,343		551,796
Net earned premiums	201,262		173,633		580,398		499,888
Net losses and loss adjustment expenses	(128,850)		(110,242)		(370,242)		(340,893)
Commission expenses	(31,258)		(25,934)		(90,211)		(80,164)
Other operating expenses	(40,112)		(34,989)		(116,238)		(107,341)
Other income (expense)	789		(921)		2,087		643
Underwriting profit (loss)	1,831		1,547		5,794		(27,867)
Net investment income	13,597		16,259		40,632		51,072
Net realized gains (losses)	4,761		2,615		10,170		3,459
Interest expense	(2,049)		(2,047)		(6,147)		(6,140)
Income (loss) before income taxes	18,140		18,374		50,449		20,524
Income tax expense (benefit)	5,225		4,476		14,731		5,015
Net income (loss)	12,915		13,898		35,718		15,509
Identifiable assets	3,994,697		3,623,375		3,994,697		3,623,375		3,670,007
Losses and loss adjustment expenses ratio	64.00%		63.50%		63.80%		68.20%
Commission expense ratio	15.50%		14.90%		15.50%		16.00%
Other operating expense ratio	19.60%	[1]	20.70%	[1]	19.70%	[1]	21.40%	[1]
Combined ratio	99.10%		99.10%		99.00%		105.60%
Insurance Companies
Segment Reporting Information [Line Items]
Gross written premiums	218,361		191,175		680,763		584,718
Net written premiums	142,148		135,292		457,463		389,236
Net earned premiums	149,090		119,332		422,215		333,139
Net losses and loss adjustment expenses	(113,303)		(76,755)		(304,483)		(228,882)
Commission expenses	(20,827)		(16,514)		(61,245)		(45,256)
Other operating expenses	(29,387)		(25,735)		(83,646)		(79,050)
Other income (expense)	1,229		554		3,750		2,871
Underwriting profit (loss)	(13,198)		882		(23,409)		(17,178)
Net investment income	12,004		14,037		34,225		44,009
Net realized gains (losses)	2,609		2,809		6,809		5,664
Income (loss) before income taxes	1,415		17,728		17,625		32,495
Income tax expense (benefit)	(377)		4,379		3,603		9,224
Net income (loss)	1,792		13,349		14,022		23,271
Identifiable assets	2,995,530		2,692,597		2,995,530		2,692,597
Losses and loss adjustment expenses ratio	76.00%		64.30%		72.10%		68.70%
Commission expense ratio	14.00%		13.80%		14.50%		13.60%
Other operating expense ratio	18.90%	[1]	21.20%	[1]	18.90%	[1]	22.90%	[1]
Combined ratio	108.90%		99.30%		105.50%		105.20%
Lloyd's Operations
Segment Reporting Information [Line Items]
Gross written premiums	80,381		64,143		284,115		245,597
Net written premiums	45,898		40,065		163,880		162,560
Net earned premiums	52,172		54,301		158,183		166,749
Net losses and loss adjustment expenses	(15,547)		(33,487)		(65,759)		(112,011)
Commission expenses	(10,911)		(9,953)		(30,735)		(36,402)
Other operating expenses	(10,725)		(9,254)		(32,592)		(28,291)
Other income (expense)	40		(942)		106		(734)
Underwriting profit (loss)	15,029		665		29,203		(10,689)
Net investment income	1,552		2,158		6,289		6,733
Net realized gains (losses)	2,152		(226)		3,361		(2,409)
Income (loss) before income taxes	18,733		2,597		38,853		(6,365)
Income tax expense (benefit)	6,525		780		13,458		(2,247)
Net income (loss)	12,208		1,817		25,395		(4,118)
Identifiable assets	958,238		887,401		958,238		887,401
Losses and loss adjustment expenses ratio	29.80%		61.70%		41.60%		67.20%
Commission expense ratio	20.90%		18.30%		19.40%		21.80%
Other operating expense ratio	20.50%	[1]	18.80%	[1]	20.50%	[1]	17.40%	[1]
Combined ratio	71.20%		98.80%		81.50%		106.40%
Corporate
Segment Reporting Information [Line Items]
Commission expenses	480	[2]	533	[2]	1,769	[2]	1,494	[2]
Other income (expense)	(480)	[2]	(533)	[2]	(1,769)	[2]	(1,494)	[2]
Net investment income	41	[2]	64	[2]	118	[2]	330	[2]
Net realized gains (losses)			32	[2]			204	[2]
Interest expense	(2,049)	[2]	(2,047)	[2]	(6,147)	[2]	(6,140)	[2]
Income (loss) before income taxes	(2,008)	[2]	(1,951)	[2]	(6,029)	[2]	(5,606)	[2]
Income tax expense (benefit)	(923)	[2]	(683)	[2]	(2,330)	[2]	(1,962)	[2]
Net income (loss)	(1,085)	[2]	(1,268)	[2]	(3,699)	[2]	(3,644)	[2]
Identifiable assets	$ 40,929	[2]	$ 43,377	[2]	$ 40,929	[2]	$ 43,377	[2]

[1] Includes Other operating expenses and Other income.

[2] Includes Corporate segment intercompany eliminations.