Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012		Dec. 31, 2012
Segment Reporting Information [Line Items]
Gross written premiums	$ 332,128		$ 322,987		$ 725,350		$ 666,136
Net written premiums	198,469		190,252		467,921		433,297
Net earned premiums	205,814		196,017		408,142		379,136
Net losses and loss adjustment expenses	(131,148)		(123,407)		(262,490)		(241,392)
Commission expenses	(28,391)		(29,503)		(54,946)		(58,953)
Other operating expenses	(40,678)		(39,819)		(81,552)		(76,126)
Other income (expense)	(915)		387		(297)		1,298
Underwriting profit (loss)	4,682		3,675		8,857		3,963
Net investment income	14,246		15,777		27,903		27,035
Net realized gains (losses)	3,345		3,721		8,117		5,409
Interest expense	(2,052)		(2,049)		(4,103)		(4,098)
Income (loss) before income taxes	20,221		21,124		40,774		32,309
Income tax expense (benefit)	6,284		6,225		12,927		9,506
Net income (loss)	13,937		14,899		27,847		22,803
Identifiable assets	4,096,510		3,939,066		4,096,510		3,939,066		4,007,670
Losses and loss adjustment expenses ratio	63.70%		63.00%		64.30%		63.70%
Commission expense ratio	13.80%		15.10%		13.50%		15.50%
Other operating expense ratio	20.20%	[1]	20.00%	[1]	20.00%	[1]	19.80%	[1]
Combined ratio	97.70%		98.10%		97.80%		99.00%
Insurance
Segment Reporting Information [Line Items]
Gross written premiums	228,565		214,064		530,193		462,402
Net written premiums	143,380		134,065		359,699		315,315
Net earned premiums	156,547		141,577		310,878		273,125
Net losses and loss adjustment expenses	(106,668)		(100,003)		(213,653)		(191,180)
Commission expenses	(20,573)		(21,117)		(39,090)		(40,418)
Other operating expenses	(29,829)		(28,914)		(59,172)		(54,259)
Other income (expense)	689		879		1,348		2,521
Underwriting profit (loss)	166		(7,578)		311		(10,211)
Net investment income	12,515		13,286		24,466		22,221
Net realized gains (losses)	3,099		2,325		7,891		4,200
Income (loss) before income taxes	15,780		8,033		32,668		16,210
Income tax expense (benefit)	4,785		2,101		10,199		3,980
Net income (loss)	10,995		5,932		22,469		12,230
Identifiable assets	3,097,754		2,933,334		3,097,754		2,933,334
Losses and loss adjustment expenses ratio	68.10%		70.60%		68.70%		70.00%
Commission expense ratio	13.10%		14.90%		12.60%		14.80%
Other operating expense ratio	18.70%	[1]	19.90%	[1]	18.60%	[1]	18.90%	[1]
Combined ratio	99.90%		105.40%		99.90%		103.70%
Lloyd's
Segment Reporting Information [Line Items]
Gross written premiums	103,563		108,923		195,157		203,734
Net written premiums	55,089		56,187		108,222		117,982
Net earned premiums	49,267		54,440		97,264		106,011
Net losses and loss adjustment expenses	(24,480)		(23,404)		(48,837)		(50,212)
Commission expenses	(8,335)		(8,938)		(16,956)		(19,824)
Other operating expenses	(10,849)		(10,905)		(22,380)		(21,867)
Other income (expense)	(1,087)		60		(545)		66
Underwriting profit (loss)	4,516		11,253		8,546		14,174
Net investment income	1,728		2,454		3,430		4,737
Net realized gains (losses)	242		1,396		222		1,209
Income (loss) before income taxes	6,486		15,103		12,198		20,120
Income tax expense (benefit)	2,206		5,207		4,242		6,933
Net income (loss)	4,280		9,896		7,956		13,187
Identifiable assets	947,736		967,862		947,736		967,862
Losses and loss adjustment expenses ratio	49.70%		43.00%		50.20%		47.40%
Commission expense ratio	16.90%		16.40%		17.40%		18.70%
Other operating expense ratio	24.20%	[1]	19.90%	[1]	23.60%	[1]	20.50%	[1]
Combined ratio	90.80%		79.30%		91.20%		86.60%
Corporate
Segment Reporting Information [Line Items]
Commission expenses	517	[2]	552	[2]	1,100	[2]	1,289	[2]
Other income (expense)	(517)	[2]	(552)	[2]	(1,100)	[2]	(1,289)	[2]
Net investment income	3	[2]	37	[2]	7	[2]	77	[2]
Net realized gains (losses)	4	[2]			4	[2]
Interest expense	(2,052)	[2]	(2,049)	[2]	(4,103)	[2]	(4,098)	[2]
Income (loss) before income taxes	(2,045)	[2]	(2,012)	[2]	(4,092)	[2]	(4,021)	[2]
Income tax expense (benefit)	(707)	[2]	(1,083)	[2]	(1,514)	[2]	(1,407)	[2]
Net income (loss)	(1,338)	[2]	(929)	[2]	(2,578)	[2]	(2,614)	[2]
Identifiable assets	$ 51,020	[2]	$ 37,870	[2]	$ 51,020	[2]	$ 37,870	[2]

[1] Includes Other operating expenses and Other income.

[2] Includes Corporate segment intercompany eliminations.