Additional Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment Reporting Information [Line Items]
Gross written premiums	$ 298,742	$ 255,318	$ 964,878	$ 830,315
Net written premiums	188,046	175,357	621,343	551,796
Net earned premiums	201,262	173,633	580,398	499,888
Marine
Segment Reporting Information [Line Items]
Gross written premiums	82,595	74,120	303,168	303,397
Net written premiums	60,554	54,829	216,755	232,562
Net earned premiums	74,594	76,461	212,940	233,609
Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	172,737	140,657	532,557	415,100
Net written premiums	95,082	93,352	311,516	249,342
Net earned premiums	95,863	74,537	282,180	200,976
Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	43,410	40,541	129,153	111,818
Net written premiums	32,410	27,176	93,072	69,892
Net earned premiums	30,805	22,635	85,278	65,303
Insurance Companies
Segment Reporting Information [Line Items]
Gross written premiums	218,361	191,175	680,763	584,718
Net written premiums	142,148	135,292	457,463	389,236
Net earned premiums	149,090	119,332	422,215	333,139
Insurance Companies | Marine
Segment Reporting Information [Line Items]
Gross written premiums	44,879	47,141	156,640	175,812
Net written premiums	32,615	34,180	107,266	130,200
Net earned premiums	40,592	41,951	111,402	124,387
Insurance Companies | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	139,948	110,975	426,494	323,994
Net written premiums	83,449	77,056	274,823	201,978
Net earned premiums	83,993	58,585	240,536	156,871
Insurance Companies | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	33,534	33,059	97,629	84,912
Net written premiums	26,084	24,056	75,374	57,058
Net earned premiums	24,505	18,796	70,277	51,881
Lloyd's Operations
Segment Reporting Information [Line Items]
Gross written premiums	80,381	64,143	284,115	245,597
Net written premiums	45,898	40,065	163,880	162,560
Net earned premiums	52,172	54,301	158,183	166,749
Lloyd's Operations | Marine
Segment Reporting Information [Line Items]
Gross written premiums	37,716	26,979	146,528	127,585
Net written premiums	27,939	20,649	109,489	102,362
Net earned premiums	34,002	34,510	101,538	109,222
Lloyd's Operations | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	32,789	29,682	106,063	91,106
Net written premiums	11,633	16,296	36,693	47,364
Net earned premiums	11,870	15,952	41,644	44,105
Lloyd's Operations | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	9,876	7,482	31,524	26,906
Net written premiums	6,326	3,120	17,698	12,834
Net earned premiums	$ 6,300	$ 3,839	$ 15,001	$ 13,422