Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013		Dec. 31, 2013
Segment Reporting Information [Line Items]
Gross written premiums	$ 348,795		$ 332,128		$ 771,585		$ 725,350
Net written premiums	231,864		198,469		543,714		467,921
Net earned premiums	231,084		205,814		456,356		408,142
Net losses and loss adjustment expenses	(140,220)		(131,148)		(275,287)		(262,490)
Commission expenses	(32,150)		(28,391)		(57,877)		(54,946)
Other operating expenses	(47,992)		(40,678)		(95,138)		(81,552)
Other underwriting income (expense)	235				390
Other income (expense)	(1,665)		(915)		8,734		(297)
Underwriting profit (loss)	10,957		4,682		28,444		8,857
Net investment income	15,648		14,246		32,258		27,903
Net realized gains (losses)	4,473		3,345		5,306		8,117
Interest expense	(4,319)		(2,052)		(8,171)		(4,103)
Other income	(1,900)				8,344
Income (loss) before income taxes	24,859		20,221		66,181		40,774
Income tax expense (benefit)	7,998		6,284		21,352		12,927
Net income (loss)	16,861		13,937		44,829		27,847
Identifiable assets	4,415,731		4,096,510		4,415,731		4,096,510		4,169,452
Losses and loss adjustment expenses ratio	60.70%		63.70%		60.30%		64.30%
Commission expense ratio	13.90%		13.80%		12.70%		13.50%
Other operating expense ratio	20.70%	[1]	20.20%	[1]	20.80%	[2]	20.00%	[1]
Combined ratio	95.30%		97.70%		93.80%		97.80%
Insurance Companies
Segment Reporting Information [Line Items]
Gross written premiums	243,729		228,565		555,527		530,193
Net written premiums	174,041		143,380		407,992		359,699
Net earned premiums	178,412		156,547		344,296		310,878
Net losses and loss adjustment expenses	(108,502)		(106,668)		(214,929)		(213,653)
Commission expenses	(23,021)		(20,573)		(39,731)		(39,090)
Other operating expenses	(34,415)		(29,829)		(67,802)		(59,172)
Other underwriting income (expense)	768				1,426
Other income (expense)			689				1,348
Underwriting profit (loss)	13,242		166		23,260		311
Net investment income	13,805		12,515		28,582		24,466
Net realized gains (losses)	4,458		3,099		5,695		7,891
Other income	(68)				(19)
Income (loss) before income taxes	31,437		15,780		57,518		32,668
Income tax expense (benefit)	10,049		4,785		18,161		10,199
Net income (loss)	21,388		10,995		39,357		22,469
Identifiable assets	3,280,303		3,097,754		3,280,303		3,097,754
Losses and loss adjustment expenses ratio	60.80%		68.10%		62.40%		68.70%
Commission expense ratio	12.90%		13.10%		11.50%		12.60%
Other operating expense ratio	18.90%	[1]	18.70%	[1]	19.30%	[2]	18.60%	[1]
Combined ratio	92.60%		99.90%		93.20%		99.90%
Lloyd's
Segment Reporting Information [Line Items]
Gross written premiums	105,066		103,563		216,058		195,157
Net written premiums	57,823		55,089		135,722		108,222
Net earned premiums	52,672		49,267		112,060		97,264
Net losses and loss adjustment expenses	(31,718)		(24,480)		(60,358)		(48,837)
Commission expenses	(9,675)		(8,335)		(19,201)		(16,956)
Other operating expenses	(13,577)		(10,849)		(27,336)		(22,380)
Other underwriting income (expense)	13				19
Other income (expense)			(1,087)				(545)
Underwriting profit (loss)	(2,285)		4,516		5,184		8,546
Net investment income	1,824		1,728		3,639		3,430
Net realized gains (losses)	15		242		(389)		222
Other income	(1,832)				8,363
Income (loss) before income taxes	(2,278)		6,486		16,797		12,198
Income tax expense (benefit)	(672)		2,206		5,966		4,242
Net income (loss)	(1,606)		4,280		10,831		7,956
Identifiable assets	985,181		947,736		985,181		947,736
Losses and loss adjustment expenses ratio	60.20%		49.70%		53.90%		50.20%
Commission expense ratio	18.40%		16.90%		17.10%		17.40%
Other operating expense ratio	25.70%	[1]	24.20%	[1]	24.40%	[2]	23.60%	[1]
Combined ratio	104.30%		90.80%		95.40%		91.20%
Corporate
Segment Reporting Information [Line Items]
Commission expenses	546	[3]	517	[3]	1,055	[3]	1,100	[3]
Other underwriting income (expense)	(546)	[3]			(1,055)	[3]
Other income (expense)			(517)	[3]			(1,100)	[3]
Net investment income	19	[3]	3	[3]	37	[3]	7	[3]
Net realized gains (losses)			4	[3]			4	[3]
Interest expense	(4,319)	[3]	(2,052)	[3]	(8,171)	[3]	(4,103)	[3]
Income (loss) before income taxes	(4,300)	[3]	(2,045)	[3]	(8,134)	[3]	(4,092)	[3]
Income tax expense (benefit)	(1,379)	[3]	(707)	[3]	(2,775)	[3]	(1,514)	[3]
Net income (loss)	(2,921)	[3]	(1,338)	[3]	(5,359)	[3]	(2,578)	[3]
Identifiable assets	$ 150,247	[3]	$ 51,020	[3]	$ 150,247	[3]	$ 51,020	[3]

[1] Includes Other operating expenses and Other income.

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

[3] Includes Corporate segment intercompany eliminations.