Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012		Dec. 31, 2012
Segment Reporting Information [Line Items]
Gross written premiums	$ 393,222		$ 343,149
Net written premiums	269,452		243,045
Net earned premiums	202,328		183,119
Net losses and loss adjustment expenses	(131,342)		(117,985)
Commission expenses	(26,555)		(29,450)
Other operating expenses	(40,874)		(36,307)
Other income (expense)	618		911
Underwriting profit (loss)	4,175		288
Net investment income	13,657		11,258
Net realized gains (losses)	4,772		1,688
Interest expense	(2,051)		(2,049)
Income (loss) before income taxes	20,553		11,185
Income tax expense (benefit)	6,643		3,281
Net income (loss)	13,910		7,904
Identifiable assets	4,071,045		3,847,527		4,007,670
Losses and loss adjustment expenses ratio	64.90%		64.40%
Commission expense ratio	13.10%		16.10%
Other operating expense ratio	19.90%	[1]	19.30%	[1]
Combined ratio	97.90%		99.80%
Insurance
Segment Reporting Information [Line Items]
Gross written premiums	301,628		248,338
Net written premiums	216,319		181,250
Net earned premiums	154,331		131,548
Net losses and loss adjustment expenses	(106,985)		(91,177)
Commission expenses	(18,517)		(19,301)
Other operating expenses	(29,343)		(25,345)
Other income (expense)	659		1,642
Underwriting profit (loss)	145		(2,633)
Net investment income	11,951		8,935
Net realized gains (losses)	4,792		1,875
Income (loss) before income taxes	16,888		8,177
Income tax expense (benefit)	5,404		2,258
Net income (loss)	11,484		5,919
Identifiable assets	3,087,308		2,899,369
Losses and loss adjustment expenses ratio	69.30%		69.30%
Commission expense ratio	12.00%		14.70%
Other operating expense ratio	18.60%	[1]	18.00%	[1]
Combined ratio	99.90%		102.00%
Lloyd's
Segment Reporting Information [Line Items]
Gross written premiums	91,594		94,811
Net written premiums	53,133		61,795
Net earned premiums	47,997		51,571
Net losses and loss adjustment expenses	(24,357)		(26,808)
Commission expenses	(8,621)		(10,886)
Other operating expenses	(11,531)		(10,962)
Other income (expense)	542		6
Underwriting profit (loss)	4,030		2,921
Net investment income	1,702		2,283
Net realized gains (losses)	(20)		(187)
Income (loss) before income taxes	5,712		5,017
Income tax expense (benefit)	2,046		1,726
Net income (loss)	3,666		3,291
Identifiable assets	936,471		907,760
Losses and loss adjustment expenses ratio	50.70%		52.00%
Commission expense ratio	18.00%		21.10%
Other operating expense ratio	22.90%	[1]	21.20%	[1]
Combined ratio	91.60%		94.30%
Corporate
Segment Reporting Information [Line Items]
Commission expenses	583	[2]	737	[2]
Other income (expense)	(583)	[2]	(737)	[2]
Net investment income	4	[2]	40	[2]
Interest expense	(2,051)	[2]	(2,049)	[2]
Income (loss) before income taxes	(2,047)	[2]	(2,009)	[2]
Income tax expense (benefit)	(807)	[2]	(703)	[2]
Net income (loss)	(1,240)	[2]	(1,306)	[2]
Identifiable assets	$ 47,266	[2]	$ 40,398	[2]

[1] Includes Other operating expenses and Other income.

[2] Includes Corporate segment intercompany eliminations.