Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Segment Reporting Information [Line Items]
Gross written premiums	$ 333,091		$ 312,076	$ 332,128	$ 393,222	$ 321,587		$ 298,742	$ 322,987	$ 343,149	$ 1,370,517		$ 1,286,465		$ 1,108,216
Net written premiums	223,445		196,556	198,469	269,452	212,312		188,046	190,252	243,045	887,922		833,655		753,798
Net earned premiums	219,902		213,895	205,814	202,328	201,566		201,262	196,017	183,119	841,939		781,964		691,645
Net losses and loss adjustment expenses	(131,385)		(125,086)	(131,148)	(131,342)	(127,191)		(128,850)	(123,407)	(117,985)	(518,961)		(497,433)		(476,997)
Commission expenses	(30,863)		(27,685)	(28,391)	(26,555)	(31,259)		(31,258)	(29,503)	(29,450)	(113,494)		(121,470)		(110,437)
Other operating expenses	(43,826)		(39,056)	(40,678)	(40,874)	(42,841)		(40,112)	(39,819)	(36,307)	(164,434)		(159,079)		(138,029)
Other income (expense)	(665)		(210)	(915)	618	(599)		789	387	911	(1,172)		1,488		1,229
Underwriting profit (loss)											43,878		5,470		(32,589)
Net investment income	14,254		14,094	14,246	13,657	13,616		13,597	15,777	11,258	56,251		54,248		63,500
Net realized gains (losses)											20,546		40,216		10,011
Call premium on Senior Notes	(17,895)										(17,895)
Interest expense	(4,351)		(2,053)	(2,052)	(2,051)	(2,051)		(2,049)	(2,049)	(2,049)	(10,507)		(8,198)		(8,188)
Income before income taxes	20,409		31,090	20,221	20,553	41,287		18,140	21,124	11,185	92,273		91,736		32,734
Income tax expense (benefit)	6,076		9,804	6,284	6,643	13,243		5,225	6,225	3,281	28,807		27,974		7,137
Net income (loss)	14,333		21,286	13,937	13,910	28,044		12,915	14,899	7,904	63,466		63,762		25,597
Identifiable assets	4,169,452					4,007,670					4,169,452		4,007,670		3,670,007
Losses and loss adjustment expenses ratio											61.60%		63.60%		69.00%
Commission expense ratio											13.50%		15.50%		16.00%
Other operating expense ratio											19.70%	[1]	20.20%	[1]	19.70%	[1]
Combined ratio	94.00%		89.80%	97.70%	97.90%	100.20%		99.10%	98.10%	99.80%	94.80%		99.30%		104.70%
Insurance
Segment Reporting Information [Line Items]
Gross written premiums											1,002,275		921,325		788,419
Net written premiums											680,008		622,956		542,391
Net earned premiums											639,338		571,439		472,463
Net losses and loss adjustment expenses											(415,413)		(417,082)		(341,625)
Commission expenses											(81,132)		(81,370)		(64,165)
Other operating expenses											(119,920)		(113,625)		(101,517)
Other income (expense)											2,764		3,790		3,955
Underwriting profit (loss)											25,637		(36,848)		(30,889)
Net investment income											49,083		46,549		54,164
Net realized gains (losses)											20,600		36,468		12,151
Income before income taxes											95,320		46,169		35,426
Income tax expense (benefit)											29,965		12,686		8,271
Net income (loss)											65,355		33,483		27,155
Identifiable assets	3,077,437					3,036,489					3,077,437		3,036,489		2,768,930
Losses and loss adjustment expenses ratio											65.00%		73.00%		72.30%
Commission expense ratio											12.70%		14.20%		13.60%
Other operating expense ratio											18.30%	[1]	19.20%	[1]	20.60%	[1]
Combined ratio											96.00%		106.40%		106.50%
Lloyd's
Segment Reporting Information [Line Items]
Gross written premiums											368,242		365,140		319,797
Net written premiums											207,914		210,699		211,407
Net earned premiums											202,601		210,525		219,182
Net losses and loss adjustment expenses											(103,548)		(80,351)		(135,372)
Commission expenses											(34,710)		(42,449)		(48,341)
Other operating expenses											(44,514)		(45,454)		(36,512)
Other income (expense)											(1,588)		47		(657)
Underwriting profit (loss)											18,241		42,318		(1,700)
Net investment income											7,160		7,551		8,955
Net realized gains (losses)											(58)		3,555		(2,354)
Income before income taxes											25,343		53,424		4,901
Income tax expense (benefit)											8,728		18,620		1,523
Net income (loss)											16,615		34,804		3,378
Identifiable assets	930,567					928,448					930,567		928,448		865,230
Losses and loss adjustment expenses ratio											51.10%		38.20%		61.80%
Commission expense ratio											17.10%		20.20%		22.10%
Other operating expense ratio											22.80%	[1]	21.50%	[1]	16.90%	[1]
Combined ratio											91.00%		79.90%		100.80%
Corporate
Segment Reporting Information [Line Items]
Commission expenses											2,348	[2]	2,349	[2]	2,069	[2]
Other income (expense)											(2,348)	[2]	(2,349)	[2]	(2,069)	[2]
Net investment income											8	[2]	148	[2]	381	[2]
Net realized gains (losses)											4	[2]	193	[2]	214	[2]
Call premium on Senior Notes											(17,895)	[2]
Interest expense											(10,507)	[2]	(8,198)	[2]	(8,188)	[2]
Income before income taxes											(28,390)	[2]	(7,857)	[2]	(7,593)	[2]
Income tax expense (benefit)											(9,886)	[2]	(3,332)	[2]	(2,657)	[2]
Net income (loss)											(18,504)	[2]	(4,525)	[2]	(4,936)	[2]
Identifiable assets	$ 161,448	[2]				$ 42,733	[2]				$ 161,448	[2]	$ 42,733	[2]	$ 35,847	[2]

[1] Includes Other operating expenses and Other income.

[2] Includes Corporate segment intercompany eliminations.