Additional Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting Information [Line Items]
Gross written premiums	$ 348,795	$ 332,128	$ 771,585	$ 725,350
Net written premiums	231,864	198,469	543,714	467,921
Gross written premiums, change	5.00%		6.40%
Net earned premiums	231,084	205,814	456,356	408,142
Net written premiums, change	16.80%		16.20%
Net earned premiums, change	12.30%		11.80%
Insurance Companies
Segment Reporting Information [Line Items]
Gross written premiums	243,729	228,565	555,527	530,193
Net written premiums	174,041	143,380	407,992	359,699
Gross written premiums, change	6.60%		4.80%
Net earned premiums	178,412	156,547	344,296	310,878
Net written premiums, change	21.40%		13.40%
Net earned premiums, change	14.00%		10.70%
Insurance Companies | Marine
Segment Reporting Information [Line Items]
Gross written premiums	48,119	43,077	100,352	93,924
Net written premiums	35,357	27,366	76,481	68,507
Gross written premiums, change	11.70%		6.80%
Net earned premiums	31,752	31,798	64,512	68,523
Net written premiums, change	29.20%		11.60%
Net earned premiums, change	(0.10%)		(5.90%)
Insurance Companies | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	168,597	152,095	397,953	371,059
Net written premiums	119,633	90,711	291,994	240,662
Gross written premiums, change	10.80%		7.20%
Net earned premiums	124,695	99,477	235,521	192,195
Net written premiums, change	31.90%		21.30%
Net earned premiums, change	25.40%		22.50%
Insurance Companies | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	27,013	33,393	57,222	65,210
Net written premiums	19,051	25,303	39,517	50,530
Gross written premiums, change	(19.10%)		(12.20%)
Net earned premiums	21,965	25,272	44,263	50,160
Net written premiums, change	(24.70%)		(21.80%)
Net earned premiums, change	(13.10%)		(11.80%)
Lloyd's
Segment Reporting Information [Line Items]
Gross written premiums	105,066	103,563	216,058	195,157
Net written premiums	57,823	55,089	135,722	108,222
Gross written premiums, change	1.50%		10.70%
Net earned premiums	52,672	49,267	112,060	97,264
Net written premiums, change	5.00%		25.40%
Net earned premiums, change	6.90%		15.20%
Lloyd's | Marine
Segment Reporting Information [Line Items]
Gross written premiums	46,005	45,893	108,962	99,537
Net written premiums	29,562	34,060	81,693	73,618
Gross written premiums, change	0.20%		9.50%
Net earned premiums	32,213	34,623	71,100	68,668
Net written premiums, change	(13.20%)		11.00%
Net earned premiums, change	(7.00%)		3.50%
Lloyd's | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	37,162	41,217	70,241	66,275
Net written premiums	14,392	12,181	30,926	19,493
Gross written premiums, change	(9.80%)		6.00%
Net earned premiums	11,463	8,562	24,421	16,441
Net written premiums, change	18.20%		58.70%
Net earned premiums, change	33.90%		48.50%
Lloyd's | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	21,899	16,453	36,855	29,345
Net written premiums	13,869	8,848	23,103	15,111
Gross written premiums, change	33.10%		25.60%
Net earned premiums	$ 8,996	$ 6,082	$ 16,539	$ 12,155
Net written premiums, change	56.70%		52.90%
Net earned premiums, change	47.90%		36.10%