Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011		Dec. 31, 2011
Segment Reporting Information [Line Items]
Gross written premiums	$ 343,149		$ 296,283
Net written premiums	243,045		193,076
Net earned premiums	183,119		152,478
Net losses and loss adjustment expenses	(117,985)		(116,788)
Commission expenses	(29,450)		(26,200)
Other operating expenses	(36,307)		(36,575)
Other income (expense)	911		991
Underwriting profit (loss)	288		(26,094)
Net investment income	11,258		17,384
Net realized gains (losses)	1,688		(1,630)
Interest expense	(2,049)		(2,046)
Income (loss) before income taxes	11,185		(12,386)
Income tax expense (benefit)	3,281		(4,493)
Net income (loss)	7,904		(7,893)
Identifiable assets	3,847,527		3,637,110		3,670,007
Losses and loss adjustment expenses ratio	64.40%		76.60%
Commission expense ratio	16.10%		17.20%
Other operating expense ratio	19.30%	[1]	23.30%	[1]
Combined ratio	99.80%		117.10%
Insurance Companies
Segment Reporting Information [Line Items]
Gross written premiums	248,338		206,776
Net written premiums	181,250		130,740
Net earned premiums	131,548		98,820
Net losses and loss adjustment expenses	(91,177)		(74,797)
Commission expenses	(19,301)		(12,340)
Other operating expenses	(25,345)		(26,799)
Other income (expense)	1,642		1,691
Underwriting profit (loss)	(2,633)		(13,425)
Net investment income	8,935		14,983
Net realized gains (losses)	1,875		(245)
Income (loss) before income taxes	8,177		1,313
Income tax expense (benefit)	2,258		228
Net income (loss)	5,919		1,085
Identifiable assets	2,899,369		2,685,679
Losses and loss adjustment expenses ratio	69.30%		75.70%
Commission expense ratio	14.70%		12.50%
Other operating expense ratio	18.00%	[1]	25.40%	[1]
Combined ratio	102.00%		113.60%
Lloyd's Operations
Segment Reporting Information [Line Items]
Gross written premiums	94,811		89,507
Net written premiums	61,795		62,336
Net earned premiums	51,571		53,658
Net losses and loss adjustment expenses	(26,808)		(41,991)
Commission expenses	(10,886)		(14,407)
Other operating expenses	(10,962)		(9,776)
Other income (expense)	6		(153)
Underwriting profit (loss)	2,921		(12,669)
Net investment income	2,283		2,255
Net realized gains (losses)	(187)		(1,385)
Income (loss) before income taxes	5,017		(11,799)
Income tax expense (benefit)	1,726		(4,056)
Net income (loss)	3,291		(7,743)
Identifiable assets	907,760		883,603
Losses and loss adjustment expenses ratio	52.00%		78.30%
Commission expense ratio	21.10%		26.80%
Other operating expense ratio	21.20%	[1]	18.50%	[1]
Combined ratio	94.30%		123.60%
Corporate
Segment Reporting Information [Line Items]
Commission expenses	737	[2]	547	[2]
Other income (expense)	(737)	[2]	(547)	[2]
Underwriting profit (loss)			0	[2]
Net investment income	40	[2]	146	[2]
Interest expense	(2,049)	[2]	(2,046)	[2]
Income (loss) before income taxes	(2,009)	[2]	(1,900)	[2]
Income tax expense (benefit)	(703)	[2]	(665)	[2]
Net income (loss)	(1,306)	[2]	(1,235)	[2]
Identifiable assets	$ 40,398	[2]	$ 67,828	[2]

[1] Includes Other operating expenses and Other income.

[2] Includes Corporate segment intercompany eliminations.