Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Segment Reporting Information [Line Items]
Gross written premiums	$ 321,587		$ 298,742	$ 322,987	$ 343,149	$ 277,901		$ 255,318	$ 278,714	$ 296,283	$ 1,286,465		$ 1,108,216		$ 987,201
Net written premiums	212,312		188,046	190,252	243,045	202,002		175,357	183,363	193,076	833,655		753,798		653,938
Net earned premiums	201,566		201,262	196,017	183,119	191,757		173,633	173,777	152,478	781,964		691,645		659,931
Net losses and loss adjustment expenses	(127,191)		(128,850)	(123,407)	(117,985)	(136,104)		(110,242)	(113,863)	(116,788)	(497,433)		(476,997)		(421,155)
Commission expenses	(31,259)		(31,258)	(29,503)	(29,450)	(30,273)		(25,934)	(28,030)	(26,200)	(121,470)		(110,437)		(109,113)
Other operating expenses	(42,841)		(40,112)	(39,819)	(36,307)	(30,688)		(34,989)	(35,777)	(36,575)	(159,079)		(138,029)		(139,700)
Other income (expense)	(599)		789	387	911	586		(921)	573	991	1,488		1,229		5,143
Underwriting profit (loss)											5,470		(32,589)		(4,894)
Net investment income	13,616		13,597	15,777	11,258	12,428		16,259	17,429	17,384	54,248		63,500		71,662
Net realized gains (losses)											40,216		10,011		40,239
Interest expense	(2,051)		(2,049)	(2,049)	(2,049)	(2,048)		(2,047)	(2,047)	(2,046)	(8,198)		(8,188)		(8,178)
Income (loss) before income taxes	41,287		18,140	21,124	11,185	12,210		18,374	14,536	(12,386)	91,736		32,734		98,829
Income tax expense (benefit)	13,243		5,225	6,225	3,281	2,122		4,476	5,032	(4,493)	27,974		7,137		29,251
Net income (loss)	28,044		12,915	14,899	7,904	10,088		13,898	9,504	(7,893)	63,762		25,597		69,578
Identifiable assets	4,007,670					3,670,007					4,007,670		3,670,007		3,531,459
Losses and loss adjustment expenses ratio											63.60%		69.00%		63.80%
Commission expense ratio											15.50%		16.00%		16.50%
Other operating expense ratio											20.20%	[1]	19.70%	[1]	20.40%	[1]
Combined ratio	100.20%		99.10%	98.10%	99.80%	102.50%		99.10%	101.90%	117.10%	99.30%		104.70%		100.70%
Insurance Companies
Segment Reporting Information [Line Items]
Gross written premiums											921,325		788,419		665,505
Net written premiums											622,956		542,391		429,355
Net earned premiums											571,439		472,463		438,851
Net losses and loss adjustment expenses											(417,082)		(341,625)		(280,120)
Commission expenses											(81,370)		(64,165)		(59,122)
Other operating expenses											(113,625)		(101,517)		(106,631)
Other income (expense)											3,790		3,955		1,698
Underwriting profit (loss)											(36,848)		(30,889)		(5,324)
Net investment income											46,549		54,164		62,792
Net realized gains (losses)											36,468		12,151		36,057
Income (loss) before income taxes											46,169		35,426		93,525
Income tax expense (benefit)											12,686		8,271		27,219
Net income (loss)											33,483		27,155		66,306
Identifiable assets	3,036,489					2,768,930					3,036,489		2,768,930		2,623,871
Losses and loss adjustment expenses ratio											73.00%		72.30%		63.80%
Commission expense ratio											14.20%		13.60%		13.50%
Other operating expense ratio											19.20%	[1]	20.60%	[1]	23.90%	[1]
Combined ratio											106.40%		106.50%		101.20%
Lloyd's Operations
Segment Reporting Information [Line Items]
Gross written premiums											365,140		319,797		321,696
Net written premiums											210,699		211,407		224,583
Net earned premiums											210,525		219,182		221,080
Net losses and loss adjustment expenses											(80,351)		(135,372)		(141,035)
Commission expenses											(42,449)		(48,341)		(49,991)
Other operating expenses											(45,454)		(36,512)		(33,112)
Other income (expense)											47		(657)		3,488
Underwriting profit (loss)											42,318		(1,700)		430
Net investment income											7,551		8,955		8,286
Net realized gains (losses)											3,555		(2,354)		3,323
Income (loss) before income taxes											53,424		4,901		12,039
Income tax expense (benefit)											18,620		1,523		4,389
Net income (loss)											34,804		3,378		7,650
Identifiable assets	928,448					865,230					928,448		865,230		838,047
Losses and loss adjustment expenses ratio											38.20%		61.80%		63.80%
Commission expense ratio											20.20%		22.10%		22.60%
Other operating expense ratio											21.50%	[1]	16.90%	[1]	13.40%	[1]
Combined ratio											79.90%		100.80%		99.80%
Corporate
Segment Reporting Information [Line Items]
Commission expenses											2,349	[2]	2,069	[2]
Other operating expenses															43	[2]
Other income (expense)											(2,349)	[2]	(2,069)	[2]	(43)	[2]
Net investment income											148	[2]	381	[2]	584	[2]
Net realized gains (losses)											193	[2]	214	[2]	859	[2]
Interest expense											(8,198)	[2]	(8,188)	[2]	(8,178)	[2]
Income (loss) before income taxes											(7,857)	[2]	(7,593)	[2]	(6,735)	[2]
Income tax expense (benefit)											(3,332)	[2]	(2,657)	[2]	(2,357)	[2]
Net income (loss)											(4,525)	[2]	(4,936)	[2]	(4,378)	[2]
Identifiable assets	$ 42,733	[2]				$ 35,847	[2]				$ 42,733	[2]	$ 35,847	[2]	$ 69,541	[2]

[1] Includes Other operating expenses and Other income.

[2] Includes Corporate segment intercompany eliminations.