Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013		Dec. 31, 2013
Segment Reporting Information [Line Items]
Gross written premiums	$ 327,469		$ 312,076		$ 1,099,054		$ 1,037,426
Net written premiums	228,417		196,556		772,131		664,477
Net earned premiums	245,367		213,895		701,723		622,037
Net losses and loss adjustment expenses	(135,284)		(125,086)		(410,571)		(387,576)
Commission expenses	(33,943)		(27,685)		(91,820)		(82,631)
Other operating expenses	(50,388)		(39,056)		(145,526)		(120,608)
Other underwriting income (expense)	107				497
Other income (expense)	1,336		(210)		10,070		(507)
Underwriting profit (loss)	25,859		21,858		54,303		30,715
Net investment income	15,839		14,094		48,097		41,997
Net realized gains (losses)	6,718		(2,809)		12,024		5,308
Interest expense	(3,388)		(2,053)		(11,559)		(6,156)
Other income	1,229				9,573
Income (loss) before income taxes	46,257		31,090		112,438		71,864
Income tax expense (benefit)	15,032		9,804		36,384		22,731
Net income (loss)	31,225		21,286		76,054		49,133
Identifiable assets	4,463,313		4,074,576		4,463,313		4,074,576		4,169,452
Losses and loss adjustment expenses ratio	55.10%		58.50%		58.50%		62.30%
Commission expense ratio	13.80%		12.90%		13.10%		13.30%
Other operating expense ratio	20.60%	[1]	18.40%	[2]	20.70%	[1]	19.50%	[2]
Combined ratio	89.50%		89.80%		92.30%		95.10%
Insurance Companies
Segment Reporting Information [Line Items]
Gross written premiums	242,482		227,609		798,009		757,802
Net written premiums	172,484		149,290		580,476		508,989
Net earned premiums	182,051		162,962		526,347		473,840
Net losses and loss adjustment expenses	(108,425)		(95,315)		(323,354)		(308,968)
Commission expenses	(22,705)		(20,039)		(62,436)		(59,129)
Other operating expenses	(35,161)		(28,510)		(102,963)		(87,682)
Other underwriting income (expense)	656				2,082
Other income (expense)			955				2,303
Underwriting profit (loss)	16,416		20,053		39,676		20,364
Net investment income	13,971		12,285		42,553		36,751
Net realized gains (losses)	6,673		(1,890)		12,368		6,001
Other income	(1,100)				(1,119)
Income (loss) before income taxes	35,960		30,448		93,478		63,116
Income tax expense (benefit)	11,489		9,355		29,650		19,554
Net income (loss)	24,471		21,093		63,828		43,562
Identifiable assets	3,332,172		3,018,100		3,332,172		3,018,100
Losses and loss adjustment expenses ratio	59.60%		58.50%		61.40%		65.20%
Commission expense ratio	12.50%		12.30%		11.90%		12.50%
Other operating expense ratio	18.90%	[1]	16.90%	[2]	19.20%	[1]	18.00%	[2]
Combined ratio	91.00%		87.70%		92.50%		95.70%
Lloyd's
Segment Reporting Information [Line Items]
Gross written premiums	84,987		84,467		301,045		279,624
Net written premiums	55,933		47,266		191,655		155,488
Net earned premiums	63,316		50,933		175,376		148,197
Net losses and loss adjustment expenses	(26,859)		(29,771)		(87,217)		(78,608)
Commission expenses	(11,788)		(8,289)		(30,989)		(25,245)
Other operating expenses	(15,227)		(10,546)		(42,563)		(32,926)
Other underwriting income (expense)	1				20
Other income (expense)			(522)				(1,067)
Underwriting profit (loss)	9,443		1,805		14,627		10,351
Net investment income	1,849		1,807		5,488		5,237
Net realized gains (losses)	45		(919)		(344)		(697)
Other income	2,329				10,692
Income (loss) before income taxes	13,666		2,693		30,463		14,891
Income tax expense (benefit)	4,787		878		10,753		5,120
Net income (loss)	8,879		1,815		19,710		9,771
Identifiable assets	969,834		983,382		969,834		983,382
Losses and loss adjustment expenses ratio	42.40%		58.50%		49.70%		53.00%
Commission expense ratio	18.60%		16.30%		17.70%		17.00%
Other operating expense ratio	24.10%	[1]	21.70%	[2]	24.30%	[1]	23.00%	[2]
Combined ratio	85.10%		96.50%		91.70%		93.00%
Corporate
Segment Reporting Information [Line Items]
Commission expenses	550	[3]	643	[3]	1,605	[3]	1,743	[3]
Other underwriting income (expense)	(550)	[3]			(1,605)	[3]
Other income (expense)			(643)	[3]			(1,743)	[3]
Net investment income	19	[3]	2	[3]	56	[3]	9	[3]
Net realized gains (losses)							4	[3]
Interest expense	(3,388)	[3]	(2,053)	[3]	(11,559)	[3]	(6,156)	[3]
Income (loss) before income taxes	(3,369)	[3]	(2,051)	[3]	(11,503)	[3]	(6,143)	[3]
Income tax expense (benefit)	(1,244)	[3]	(429)	[3]	(4,019)	[3]	(1,943)	[3]
Net income (loss)	(2,125)	[3]	(1,622)	[3]	(7,484)	[3]	(4,200)	[3]
Identifiable assets	$ 161,307	[3]	$ 73,094	[3]	$ 161,307	[3]	$ 73,094	[3]

[1] Includes Other operating expenses and Other underwriting income.

[2] Includes Other operating expenses and Other income.

[3] Includes Corporate segment intercompany eliminations.