Additional Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 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
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
Insurance Companies | Marine
Segment Reporting Information [Line Items]
Gross written premiums	52,233	50,847
Net written premiums	41,124	41,141
Net earned premiums	32,760	36,725
Insurance Companies | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	229,356	218,964
Net written premiums	172,361	149,951
Net earned premiums	110,826	92,718
Insurance Companies | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	30,209	31,817
Net written premiums	20,466	25,227
Net earned premiums	22,298	24,888
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
Lloyd's | Marine
Segment Reporting Information [Line Items]
Gross written premiums	62,957	53,644
Net written premiums	52,131	39,558
Net earned premiums	38,887	34,045
Lloyd's | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	33,079	25,058
Net written premiums	16,534	7,312
Net earned premiums	12,958	7,879
Lloyd's | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	14,956	12,892
Net written premiums	9,234	6,263
Net earned premiums	$ 7,543	$ 6,073