Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Segment Reporting Information [Line Items]
Gross written premiums	$ 333,299		$ 327,469	$ 348,795	$ 422,790	$ 333,091		$ 312,076	$ 332,128	$ 393,222	$ 1,432,353		$ 1,370,517		$ 1,286,465
Net written premiums	228,007		228,417	231,864	311,850	223,445		196,556	198,469	269,452	1,000,138		887,922		833,655
Net earned premiums	234,172		245,367	231,084	225,272	219,902		213,895	205,814	202,328	935,895		841,939		781,964
Net losses and loss adjustment expenses	(134,658)		(135,284)	(140,220)	(135,067)	(131,385)		(125,086)	(131,148)	(131,342)	(545,229)		(518,961)		(497,433)
Commission expenses	(33,708)		(33,943)	(32,150)	(25,727)	(30,863)		(27,685)	(28,391)	(26,555)	(125,528)		(113,494)		(121,470)
Other operating expenses	(51,299)		(50,388)	(47,992)	(47,146)	(43,826)		(39,056)	(40,678)	(40,874)	(196,825)		(164,434)		(159,079)
Other underwriting income (expense)											595
Other income (expense)	586		1,336	(1,665)	10,399	(665)		(210)	(915)	618	10,656		(1,172)		1,488
Underwriting profit (loss)											68,908		43,878		5,470
Net investment income	16,071		15,839	15,648	16,610	14,254		14,094	14,246	13,657	64,168		56,251		54,248
Net realized gains (losses)											12,812		20,546		40,216
Call premium on Senior Notes						(17,895)							(17,895)
Interest expense	(3,854)		(3,388)	(4,319)	(3,852)	(4,351)		(2,053)	(2,052)	(2,051)	(15,413)		(10,507)		(8,198)
Other income (expense)											10,061
Income (loss) before income taxes	28,098		46,257	24,859	41,322	20,409		31,090	20,221	20,553	140,536		92,273		91,736
Income tax expense (benefit)	8,823		15,032	7,998	13,354	6,076		9,804	6,284	6,643	45,207		28,807		27,974
Net income (loss)	19,275		31,225	16,861	27,968	14,333		21,286	13,937	13,910	95,329		63,466		63,762
Identifiable assets	4,464,176					4,169,452					4,464,176		4,169,452		4,007,670
Losses and loss adjustment expenses ratio											58.30%		61.60%		63.60%
Commission expense ratio											13.40%		13.50%		15.50%
Other operating expense ratio											20.90%	[1]	19.70%	[2]	20.20%	[2]
Combined ratio	93.80%		89.50%	95.30%	92.20%	94.00%		89.80%	97.70%	97.90%	92.60%		94.80%		99.30%
Insurance Companies
Segment Reporting Information [Line Items]
Gross written premiums											1,045,454		1,002,275		921,325
Net written premiums											752,773		680,008		622,956
Net earned premiums											704,574		639,338		571,439
Net losses and loss adjustment expenses											(434,396)		(415,413)		(417,082)
Commission expenses											(85,137)		(81,132)		(81,370)
Other operating expenses											(138,675)		(119,920)		(113,625)
Other underwriting income (expense)											2,727
Other income (expense)													2,764		3,790
Underwriting profit (loss)											49,093		25,637		(36,848)
Net investment income											56,714		49,083		46,549
Net realized gains (losses)											12,715		20,600		36,468
Other income (expense)											(2,182)
Income (loss) before income taxes											116,340		95,320		46,169
Income tax expense (benefit)											36,609		29,965		12,686
Net income (loss)											79,731		65,355		33,483
Identifiable assets	3,344,084					3,077,437					3,344,084		3,077,437		3,036,489
Losses and loss adjustment expenses ratio											61.70%		65.00%		73.00%
Commission expense ratio											12.10%		12.70%		14.20%
Other operating expense ratio											19.20%	[1]	18.30%	[2]	19.20%	[2]
Combined ratio											93.00%		96.00%		106.40%
Lloyd's
Segment Reporting Information [Line Items]
Gross written premiums											386,899		368,242		365,140
Net written premiums											247,365		207,914		210,699
Net earned premiums											231,321		202,601		210,525
Net losses and loss adjustment expenses											(110,833)		(103,548)		(80,351)
Commission expenses											(42,558)		(34,710)		(42,449)
Other operating expenses											(58,150)		(44,514)		(45,454)
Other underwriting income (expense)											35
Other income (expense)													(1,588)		47
Underwriting profit (loss)											19,815		18,241		42,318
Net investment income											7,378		7,160		7,551
Net realized gains (losses)											97		(58)		3,555
Other income (expense)											12,243
Income (loss) before income taxes											39,533		25,343		53,424
Income tax expense (benefit)											13,885		8,728		18,620
Net income (loss)											25,648		16,615		34,804
Identifiable assets	957,795					930,567					957,795		930,567		928,448
Losses and loss adjustment expenses ratio											47.90%		51.10%		38.20%
Commission expense ratio											18.40%		17.10%		20.20%
Other operating expense ratio											25.10%	[1]	22.80%	[2]	21.50%	[2]
Combined ratio											91.40%		91.00%		79.90%
Corporate
Segment Reporting Information [Line Items]
Commission expenses											2,167	[3]	2,348	[3]	2,349	[3]
Other underwriting income (expense)											(2,167)	[3]
Other income (expense)													(2,348)	[3]	(2,349)	[3]
Net investment income											76	[3]	8	[3]	148	[3]
Net realized gains (losses)													4	[3]	193	[3]
Call premium on Senior Notes													(17,895)	[3]
Interest expense											(15,413)	[3]	(10,507)	[3]	(8,198)	[3]
Income (loss) before income taxes											(15,337)	[3]	(28,390)	[3]	(7,857)	[3]
Income tax expense (benefit)											(5,287)	[3]	(9,886)	[3]	(3,332)	[3]
Net income (loss)											(10,050)	[3]	(18,504)	[3]	(4,525)	[3]
Identifiable assets	$ 162,297	[3]				$ 161,448	[3]				$ 162,297	[3]	$ 161,448	[3]	$ 42,733	[3]

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

[2] Includes Other operating expenses and Other income.

[3] Includes Corporate segment intercompany eliminations.