Segment Results (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010		Dec. 31, 2010
Segment Reporting Information [Line Items]
Gross written premiums	$ 255,318		$ 233,638		$ 830,315		$ 757,351
Net written premiums	175,357		157,807		551,796		512,129
Net earned premiums	173,633		168,233		499,888		493,773
Net losses and loss adjustment expenses	(110,242)		(107,463)		(340,893)		(311,133)
Commission expenses	(25,934)		(25,185)		(80,164)		(76,178)
Other operating expenses	(34,989)		(34,699)		(107,341)		(103,819)
Other income (expense)	(921)		2,784		643		2,976
Underwriting profit (loss)	1,547		3,670		(27,867)		5,619
Net investment income	16,259		17,839		51,072		53,664
Net realized gains (losses)	2,615		3,852		3,459		20,749
Interest expense	(2,047)		(2,045)		(6,140)		(6,133)
Income (loss) before income taxes	18,374		23,316		20,524		73,899
Income tax expense (benefit)	4,476		7,091		5,015		21,659
Net income (loss)	13,898		16,225		15,509		52,240
Identifiable assets	3,623,375		3,520,996		3,623,375		3,520,996		3,531,459
Losses and loss adjustment expenses ratio	63.50%		63.90%		68.20%		63.00%
Commission expense ratio	14.90%		15.00%		16.00%		15.40%
Other operating expense ratio	20.70%	[1]	18.90%	[1]	21.40%	[1]	20.50%	[2]
Combined ratio	99.10%		97.80%		105.60%		98.90%
Insurance Companies
Segment Reporting Information [Line Items]
Gross written premiums	191,175		163,343		584,718		511,822
Net written premiums	135,292		107,916		389,236		340,657
Net earned premiums	119,332		112,198		333,139		333,834
Net losses and loss adjustment expenses	(76,755)		(72,306)		(228,882)		(205,571)
Commission expenses	(16,514)		(14,374)		(45,256)		(43,351)
Other operating expenses	(25,735)		(26,398)		(79,050)		(79,658)
Other income (expense)	554		1,380		2,871		289
Underwriting profit (loss)	882		500		(17,178)		5,543
Net investment income	14,037		15,736		44,009		47,040
Net realized gains (losses)	2,809		4,206		5,664		20,140
Income (loss) before income taxes	17,728		20,442		32,495		72,723
Income tax expense (benefit)	4,379		6,049		9,224		21,166
Net income (loss)	13,349		14,393		23,271		51,557
Identifiable assets	2,692,597		2,603,090		2,692,597		2,603,090
Losses and loss adjustment expenses ratio	64.30%		64.40%		68.70%		61.60%
Commission expense ratio	13.80%		12.80%		13.60%		13.00%
Other operating expense ratio	21.20%	[1]	22.40%	[1]	22.90%	[1]	23.70%	[2]
Combined ratio	99.30%		99.60%		105.20%		98.30%
Lloyd's Operations
Segment Reporting Information [Line Items]
Gross written premiums	64,143		70,295		245,597		245,529
Net written premiums	40,065		49,891		162,560		171,472
Net earned premiums	54,301		56,035		166,749		159,939
Net losses and loss adjustment expenses	(33,487)		(35,157)		(112,011)		(105,562)
Commission expenses	(9,953)		(10,459)		(36,402)		(32,827)
Other operating expenses	(9,254)		(8,301)		(28,291)		(24,161)
Other income (expense)	(942)		1,052		(734)		2,687
Underwriting profit (loss)	665		3,170		(10,689)		76
Net investment income	2,158		1,982		6,733		6,179
Net realized gains (losses)	(226)		(354)		(2,409)		378
Income (loss) before income taxes	2,597		4,798		(6,365)		6,633
Income tax expense (benefit)	780		1,715		(2,247)		2,403
Net income (loss)	1,817		3,083		(4,118)		4,230
Identifiable assets	887,401		841,926		887,401		841,926
Losses and loss adjustment expenses ratio	61.70%		62.70%		67.20%		66.00%
Commission expense ratio	18.30%		18.70%		21.80%		20.50%
Other operating expense ratio	18.80%	[1]	12.90%	[1]	17.40%	[1]	13.50%	[2]
Combined ratio	98.80%		94.30%		106.40%		100.00%
Corporate
Segment Reporting Information [Line Items]
Commission expenses	533	[2]	(352)	[2]	1,494	[2]
Other income (expense)	(533)	[2]	352	[2]	(1,494)	[2]
Underwriting profit (loss)	0	[2]			0	[2]
Net investment income	64	[2]	121	[2]	330	[2]	445	[2]
Net realized gains (losses)	32	[2]			204	[2]	231	[2]
Interest expense	(2,047)	[2]	(2,045)	[2]	(6,140)	[2]	(6,133)	[2]
Income (loss) before income taxes	(1,951)	[2]	(1,924)	[2]	(5,606)	[2]	(5,457)	[2]
Income tax expense (benefit)	(683)	[2]	(673)	[2]	(1,962)	[2]	(1,910)	[2]
Net income (loss)	(1,268)	[2]	(1,251)	[2]	(3,644)	[2]	(3,547)	[2]
Identifiable assets	$ 43,377	[2]	$ 75,980	[2]	$ 43,377	[2]	$ 75,980	[2]

[1] Includes Other operating expenses and Other income.

[2] Includes Corporate segment intercompany eliminations.