Financial Data by Segment (Detail) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Segment Reporting Information [Line Items]
Gross written premiums	$ 255,318	$ 233,638	$ 830,315	$ 757,351
Net written premiums	175,357	157,807	551,796	512,129
Net earned premiums	173,633	168,233	499,888	493,773
Insurance Companies
Segment Reporting Information [Line Items]
Gross written premiums	191,175	163,343	584,718	511,822
Net written premiums	135,292	107,916	389,236	340,657
Net earned premiums	119,332	112,198	333,139	333,834
Insurance Companies | Marine
Segment Reporting Information [Line Items]
Gross written premiums	47,141	49,406	175,812	172,136
Net written premiums	34,180	35,546	130,200	123,702
Net earned premiums	41,951	41,091	124,387	122,739
Insurance Companies | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	110,975	81,351	323,994	242,494
Net written premiums	77,056	52,677	201,978	156,674
Net earned premiums	58,585	50,976	156,871	152,228
Insurance Companies | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	33,059	32,586	84,912	97,192
Net written premiums	24,056	19,693	57,058	60,281
Net earned premiums	18,796	20,131	51,881	58,867
Lloyd's Operations
Segment Reporting Information [Line Items]
Gross written premiums	64,143	70,295	245,597	245,529
Net written premiums	40,065	49,891	162,560	171,472
Net earned premiums	54,301	56,035	166,749	159,939
Lloyd's Operations | Marine
Segment Reporting Information [Line Items]
Gross written premiums	26,979	32,788	127,585	133,758
Net written premiums	20,649	27,142	102,362	111,205
Net earned premiums	34,510	38,254	109,222	108,541
Lloyd's Operations | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	29,682	27,687	91,106	76,768
Net written premiums	16,296	17,414	47,364	43,049
Net earned premiums	15,952	12,202	44,105	34,880
Lloyd's Operations | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	7,482	9,820	26,906	35,003
Net written premiums	3,120	5,335	12,834	17,218
Net earned premiums	3,839	5,579	13,422	16,518
Marine
Segment Reporting Information [Line Items]
Gross written premiums	74,120	82,194	303,397	305,894
Net written premiums	54,829	62,688	232,562	234,907
Net earned premiums	76,461	79,345	233,609	231,280
Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	140,657	109,038	415,100	319,262
Net written premiums	93,352	70,091	249,342	199,723
Net earned premiums	74,537	63,178	200,976	187,108
Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	40,541	42,406	111,818	132,195
Net written premiums	27,176	25,028	69,892	77,499
Net earned premiums	$ 22,635	$ 25,710	$ 65,303	$ 75,385