Additional Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Segment Reporting Information [Line Items]
Gross written premiums	$ 393,222	$ 343,149
Net written premiums	269,452	243,045
Net earned premiums	202,328	183,119
Marine
Segment Reporting Information [Line Items]
Gross written premiums	104,491	123,640
Net written premiums	80,699	90,923
Net earned premiums	70,770	68,539
Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	244,022	180,215
Net written premiums	157,263	123,887
Net earned premiums	100,597	88,870
Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	44,709	39,294
Net written premiums	31,490	28,235
Net earned premiums	30,961	25,710
Insurance
Segment Reporting Information [Line Items]
Gross written premiums	301,628	248,338
Net written premiums	216,319	181,250
Net earned premiums	154,331	131,548
Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums	50,847	61,865
Net written premiums	41,141	42,865
Net earned premiums	36,725	35,275
Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	218,964	155,919
Net written premiums	149,951	114,532
Net earned premiums	92,718	74,368
Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	31,817	30,554
Net written premiums	25,227	23,853
Net earned premiums	24,888	21,905
Lloyd's
Segment Reporting Information [Line Items]
Gross written premiums	91,594	94,811
Net written premiums	53,133	61,795
Net earned premiums	47,997	51,571
Lloyd's | Marine
Segment Reporting Information [Line Items]
Gross written premiums	53,644	61,775
Net written premiums	39,558	48,058
Net earned premiums	34,045	33,264
Lloyd's | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	25,058	24,296
Net written premiums	7,312	9,355
Net earned premiums	7,879	14,502
Lloyd's | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	12,892	8,740
Net written premiums	6,263	4,382
Net earned premiums	$ 6,073	$ 3,805