Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013		Dec. 31, 2013
Segment Reporting Information [Line Items]
Gross written premiums	$ 422,790		$ 393,222
Net written premiums	311,850		269,452
Net earned premiums	225,272		202,328
Net losses and loss adjustment expenses	(135,067)		(131,342)
Commission expenses	(25,727)		(26,555)
Other operating expenses	(47,146)		(40,874)
Other underwriting income (expense)	155
Other income (expense)	10,399		618
Underwriting profit (loss)	17,487		4,175
Net investment income	16,610		13,657
Net realized gains (losses)	833		4,772
Interest expense	(3,852)		(2,051)
Other income	10,244
Income (loss) before income taxes	41,322		20,553
Income tax expense (benefit)	13,354		6,643
Net income (loss)	27,968		13,910
Identifiable assets	4,299,805		4,071,045		4,169,452
Losses and loss adjustment expenses ratio	60.00%		64.90%
Commission expense ratio	11.40%		13.10%
Other operating expense ratio	20.80%	[1]	19.90%	[2]
Combined ratio	92.20%		97.90%
Insurance Companies
Segment Reporting Information [Line Items]
Gross written premiums	311,798		301,628
Net written premiums	233,951		216,319
Net earned premiums	165,884		154,331
Net losses and loss adjustment expenses	(106,427)		(106,985)
Commission expenses	(16,710)		(18,517)
Other operating expenses	(33,361)		(29,343)
Other underwriting income (expense)	658
Other income (expense)			659
Underwriting profit (loss)	10,044		145
Net investment income	14,777		11,951
Net realized gains (losses)	1,237		4,792
Other income	49
Income (loss) before income taxes	26,107		16,888
Income tax expense (benefit)	8,112		5,404
Net income (loss)	17,995		11,484
Identifiable assets	3,203,763		3,087,308
Losses and loss adjustment expenses ratio	64.20%		69.30%
Commission expense ratio	10.10%		12.00%
Other operating expense ratio	19.60%	[1]	18.60%	[2]
Combined ratio	93.90%		99.90%
Lloyd's
Segment Reporting Information [Line Items]
Gross written premiums	110,992		91,594
Net written premiums	77,899		53,133
Net earned premiums	59,388		47,997
Net losses and loss adjustment expenses	(28,640)		(24,357)
Commission expenses	(9,526)		(8,621)
Other operating expenses	(13,785)		(11,531)
Other underwriting income (expense)	6
Other income (expense)			542
Underwriting profit (loss)	7,443		4,030
Net investment income	1,815		1,702
Net realized gains (losses)	(404)		(20)
Other income	10,195
Income (loss) before income taxes	19,049		5,712
Income tax expense (benefit)	6,638		2,046
Net income (loss)	12,411		3,666
Identifiable assets	941,426		936,471
Losses and loss adjustment expenses ratio	48.20%		50.70%
Commission expense ratio	16.00%		18.00%
Other operating expense ratio	23.30%	[1]	22.90%	[2]
Combined ratio	87.50%		91.60%
Corporate
Segment Reporting Information [Line Items]
Commission expenses	509	[3]	583	[3]
Other underwriting income (expense)	(509)	[3]
Other income (expense)			(583)	[3]
Net investment income	18	[3]	4	[3]
Interest expense	(3,852)	[3]	(2,051)	[3]
Income (loss) before income taxes	(3,834)	[3]	(2,047)	[3]
Income tax expense (benefit)	(1,396)	[3]	(807)	[3]
Net income (loss)	(2,438)	[3]	(1,240)	[3]
Identifiable assets	$ 154,616	[3]	$ 47,266	[3]

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

[2] Includes Other operating expenses and Other income.

[3] Includes Corporate segment intercompany eliminations.