Additional Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Gross written premiums	$ 312,076	$ 298,742	$ 1,037,426	$ 964,878
Net written premiums	196,556	188,046	664,477	621,343
Net earned premiums	213,895	201,262	622,037	580,398
Marine
Segment Reporting Information [Line Items]
Gross written premiums	74,921	82,595	268,382	303,168
Net written premiums	53,232	60,554	195,357	216,755
Net earned premiums	65,754	74,594	202,945	212,940
Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	192,097	172,737	629,431	532,557
Net written premiums	111,314	95,082	371,469	311,516
Net earned premiums	116,231	95,863	324,867	282,180
Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	45,058	43,410	139,613	129,153
Net written premiums	32,010	32,410	97,651	93,072
Net earned premiums	31,910	30,805	94,225	85,278
Insurance
Segment Reporting Information [Line Items]
Gross written premiums	227,609	218,361	757,802	680,763
Net written premiums	149,290	142,148	508,989	457,463
Net earned premiums	162,962	149,090	473,840	422,215
Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums	38,912	44,879	132,836	156,640
Net written premiums	26,437	32,615	94,944	107,266
Net earned premiums	31,490	40,592	100,013	111,402
Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	156,228	139,948	527,287	426,494
Net written premiums	97,553	83,449	338,215	274,823
Net earned premiums	105,759	83,993	297,954	240,536
Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	32,469	33,534	97,679	97,629
Net written premiums	25,300	26,084	75,830	75,374
Net earned premiums	25,713	24,505	75,873	70,277
Lloyd's
Segment Reporting Information [Line Items]
Gross written premiums	84,467	80,381	279,624	284,115
Net written premiums	47,266	45,898	155,488	163,880
Net earned premiums	50,933	52,172	148,197	158,183
Lloyd's | Marine
Segment Reporting Information [Line Items]
Gross written premiums	36,009	37,716	135,546	146,528
Net written premiums	26,795	27,939	100,413	109,489
Net earned premiums	34,264	34,002	102,932	101,538
Lloyd's | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	35,869	32,789	102,144	106,063
Net written premiums	13,761	11,633	33,254	36,693
Net earned premiums	10,472	11,870	26,913	41,644
Lloyd's | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	12,589	9,876	41,934	31,524
Net written premiums	6,710	6,326	21,821	17,698
Net earned premiums	$ 6,197	$ 6,300	$ 18,352	$ 15,001