Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012		Dec. 31, 2012
Segment Reporting Information [Line Items]
Gross written premiums	$ 312,076		$ 298,742		$ 1,037,426		$ 964,878
Net written premiums	196,556		188,046		664,477		621,343
Net earned premiums	213,895		201,262		622,037		580,398
Net losses and loss adjustment expenses	(125,086)		(128,850)		(387,576)		(370,242)
Commission expenses	(27,685)		(31,258)		(82,631)		(90,211)
Other operating expenses	(39,056)		(40,112)		(120,608)		(116,238)
Other income (expense)	(210)		789		(507)		2,087
Underwriting profit (loss)	21,858		1,831		30,715		5,794
Net investment income	14,094		13,597		41,997		40,632
Net realized gains (losses)	(2,809)		4,761		5,308		10,170
Interest expense	(2,053)		(2,049)		(6,156)		(6,147)
Income (loss) before income taxes	31,090		18,140		71,864		50,449
Income tax expense (benefit)	9,804		5,225		22,731		14,731
Net income (loss)	21,286		12,915		49,133		35,718
Identifiable assets	4,074,576		3,994,697		4,074,576		3,994,697		4,007,670
Losses and loss adjustment expenses ratio	58.50%		64.00%		62.30%		63.80%
Commission expense ratio	12.90%		15.50%		13.30%		15.50%
Other operating expense ratio	18.40%	[1]	19.60%	[1]	19.50%	[1]	19.70%	[1]
Combined ratio	89.80%		99.10%		95.10%		99.00%
Insurance
Segment Reporting Information [Line Items]
Gross written premiums	227,609		218,361		757,802		680,763
Net written premiums	149,290		142,148		508,989		457,463
Net earned premiums	162,962		149,090		473,840		422,215
Net losses and loss adjustment expenses	(95,315)		(113,303)		(308,968)		(304,483)
Commission expenses	(20,039)		(20,827)		(59,129)		(61,245)
Other operating expenses	(28,510)		(29,387)		(87,682)		(83,646)
Other income (expense)	955		1,229		2,303		3,750
Underwriting profit (loss)	20,053		(13,198)		20,364		(23,409)
Net investment income	12,285		12,004		36,751		34,225
Net realized gains (losses)	(1,890)		2,609		6,001		6,809
Income (loss) before income taxes	30,448		1,415		63,116		17,625
Income tax expense (benefit)	9,355		(377)		19,554		3,603
Net income (loss)	21,093		1,792		43,562		14,022
Identifiable assets	3,018,100		2,995,530		3,018,100		2,995,530
Losses and loss adjustment expenses ratio	58.50%		76.00%		65.20%		72.10%
Commission expense ratio	12.30%		14.00%		12.50%		14.50%
Other operating expense ratio	16.90%	[1]	18.90%	[1]	18.00%	[1]	18.90%	[1]
Combined ratio	87.70%		108.90%		95.70%		105.50%
Lloyd's
Segment Reporting Information [Line Items]
Gross written premiums	84,467		80,381		279,624		284,115
Net written premiums	47,266		45,898		155,488		163,880
Net earned premiums	50,933		52,172		148,197		158,183
Net losses and loss adjustment expenses	(29,771)		(15,547)		(78,608)		(65,759)
Commission expenses	(8,289)		(10,911)		(25,245)		(30,735)
Other operating expenses	(10,546)		(10,725)		(32,926)		(32,592)
Other income (expense)	(522)		40		(1,067)		106
Underwriting profit (loss)	1,805		15,029		10,351		29,203
Net investment income	1,807		1,552		5,237		6,289
Net realized gains (losses)	(919)		2,152		(697)		3,361
Income (loss) before income taxes	2,693		18,733		14,891		38,853
Income tax expense (benefit)	878		6,525		5,120		13,458
Net income (loss)	1,815		12,208		9,771		25,395
Identifiable assets	983,382		958,238		983,382		958,238
Losses and loss adjustment expenses ratio	58.50%		29.80%		53.00%		41.60%
Commission expense ratio	16.30%		20.90%		17.00%		19.40%
Other operating expense ratio	21.70%	[1]	20.50%	[1]	23.00%	[1]	20.50%	[1]
Combined ratio	96.50%		71.20%		93.00%		81.50%
Corporate
Segment Reporting Information [Line Items]
Commission expenses	643	[2]	480	[2]	1,743	[2]	1,769	[2]
Other income (expense)	(643)	[2]	(480)	[2]	(1,743)	[2]	(1,769)	[2]
Net investment income	2	[2]	41	[2]	9	[2]	118	[2]
Net realized gains (losses)					4	[2]
Interest expense	(2,053)	[2]	(2,049)	[2]	(6,156)	[2]	(6,147)	[2]
Income (loss) before income taxes	(2,051)	[2]	(2,008)	[2]	(6,143)	[2]	(6,029)	[2]
Income tax expense (benefit)	(429)	[2]	(923)	[2]	(1,943)	[2]	(2,330)	[2]
Net income (loss)	(1,622)	[2]	(1,085)	[2]	(4,200)	[2]	(3,699)	[2]
Identifiable assets	$ 73,094	[2]	$ 40,929	[2]	$ 73,094	[2]	$ 40,929	[2]

[1] Includes Other operating expenses and Other income.

[2] Includes Corporate segment intercompany eliminations.