Additional Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 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
Marine
Segment Reporting Information [Line Items]
Gross written premiums	124,195	131,503
Net written premiums	91,390	103,889
Net earned premiums	69,884	77,537
Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	179,660	132,190
Net written premiums	123,420	71,293
Net earned premiums	87,525	54,829
Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	39,294	32,590
Net written premiums	28,235	17,894
Net earned premiums	25,710	20,112
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
Insurance Companies | Marine
Segment Reporting Information [Line Items]
Gross written premiums	61,865	70,348
Net written premiums	42,865	54,218
Net earned premiums	35,275	40,559
Insurance Companies | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	155,919	112,888
Net written premiums	114,532	62,907
Net earned premiums	74,368	42,935
Insurance Companies | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	30,554	23,540
Net written premiums	23,853	13,615
Net earned premiums	21,905	15,326
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
Lloyd's Operations | Marine
Segment Reporting Information [Line Items]
Gross written premiums	62,330	61,155
Net written premiums	48,525	49,671
Net earned premiums	34,609	36,978
Lloyd's Operations | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	23,741	19,302
Net written premiums	8,888	8,386
Net earned premiums	13,157	11,894
Lloyd's Operations | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	8,740	9,050
Net written premiums	4,382	4,279
Net earned premiums	$ 3,805	$ 4,786