Additional Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information [Line Items]
Gross written premiums	$ 327,469	$ 312,076	$ 1,099,054	$ 1,037,426
Net written premiums	228,417	196,556	772,131	664,477
Gross written premiums, change	4.90%		5.90%
Net earned premiums	245,367	213,895	701,723	622,037
Net written premiums, change	16.20%		16.20%
Net earned premiums, change	14.70%		12.80%
Insurance Companies
Segment Reporting Information [Line Items]
Gross written premiums	242,482	227,609	798,009	757,802
Net written premiums	172,484	149,290	580,476	508,989
Gross written premiums, change	6.50%		5.30%
Net earned premiums	182,051	162,962	526,347	473,840
Net written premiums, change	15.50%		14.00%
Net earned premiums, change	11.70%		11.10%
Insurance Companies | Marine
Segment Reporting Information [Line Items]
Gross written premiums	38,726	38,912	139,078	132,836
Net written premiums	24,166	26,437	100,647	94,944
Gross written premiums, change	(0.50%)		4.70%
Net earned premiums	31,123	31,490	95,635	100,013
Net written premiums, change	(8.60%)		6.00%
Net earned premiums, change	(1.20%)		(4.40%)
Insurance Companies | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	174,868	156,228	572,821	527,287
Net written premiums	126,665	97,553	418,659	338,215
Gross written premiums, change	11.90%		8.60%
Net earned premiums	129,547	105,759	365,068	297,954
Net written premiums, change	29.80%		23.80%
Net earned premiums, change	22.50%		22.50%
Insurance Companies | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	28,888	32,469	86,110	97,679
Net written premiums	21,653	25,300	61,170	75,830
Gross written premiums, change	(11.00%)		(11.80%)
Net earned premiums	21,381	25,713	65,644	75,873
Net written premiums, change	(14.40%)		(19.30%)
Net earned premiums, change	(16.80%)		(13.50%)
Lloyd's
Segment Reporting Information [Line Items]
Gross written premiums	84,987	84,467	301,045	279,624
Net written premiums	55,933	47,266	191,655	155,488
Gross written premiums, change	0.60%		7.70%
Net earned premiums	63,316	50,933	175,376	148,197
Net written premiums, change	18.30%		23.30%
Net earned premiums, change	24.30%		18.30%
Lloyd's | Marine
Segment Reporting Information [Line Items]
Gross written premiums	38,568	36,009	147,530	135,546
Net written premiums	32,245	26,795	113,938	100,413
Gross written premiums, change	7.10%		8.80%
Net earned premiums	39,368	34,264	110,468	102,932
Net written premiums, change	20.30%		13.50%
Net earned premiums, change	14.90%		7.30%
Lloyd's | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	31,172	35,869	101,413	102,144
Net written premiums	13,758	13,761	44,684	33,254
Gross written premiums, change	(13.10%)		(0.70%)
Net earned premiums	12,980	10,472	37,401	26,913
Net written premiums, change	0.00%		34.40%
Net earned premiums, change	23.90%		39.00%
Lloyd's | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	15,247	12,589	52,102	41,934
Net written premiums	9,930	6,710	33,033	21,821
Gross written premiums, change	21.10%		24.20%
Net earned premiums	$ 10,968	$ 6,197	$ 27,507	$ 18,352
Net written premiums, change	48.00%		51.40%
Net earned premiums, change	77.00%		49.90%