Segment Revenue (Detail) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Segment Reporting Information [Line Items]
Gross written premiums	$ 278,714	$ 253,568	$ 574,997	$ 523,713
Net written premiums	183,363	165,005	376,439	354,322
Net earned premiums	173,777	161,471	326,255	325,540
Insurance Companies
Segment Reporting Information [Line Items]
Gross written premiums	186,767	170,641	393,543	348,479
Net written premiums	123,204	111,401	253,944	232,741
Net earned premiums	114,987	110,425	213,807	221,636
Insurance Companies | Marine
Segment Reporting Information [Line Items]
Gross written premiums	58,323	55,204	128,671	122,730
Net written premiums	41,802	37,153	96,020	88,156
Net earned premiums	41,877	40,554	82,436	81,648
Insurance Companies | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	100,131	81,797	213,019	161,143
Net written premiums	62,015	54,300	124,922	103,997
Net earned premiums	55,351	50,171	98,286	101,252
Insurance Companies | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	28,313	33,640	51,853	64,606
Net written premiums	19,387	19,948	33,002	40,588
Net earned premiums	17,759	19,700	33,085	38,736
Lloyd's Operations
Segment Reporting Information [Line Items]
Gross written premiums	91,947	82,927	181,454	175,234
Net written premiums	60,159	53,604	122,495	121,581
Net earned premiums	58,790	51,046	112,448	103,904
Lloyd's Operations | Marine
Segment Reporting Information [Line Items]
Gross written premiums	39,451	41,829	100,606	100,970
Net written premiums	32,042	34,421	81,713	84,063
Net earned premiums	37,734	34,727	74,712	70,287
Lloyd's Operations | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	42,122	29,122	61,424	49,081
Net written premiums	22,682	13,924	31,068	25,635
Net earned premiums	16,259	10,763	28,153	22,678
Lloyd's Operations | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	10,374	11,976	19,424	25,183
Net written premiums	5,435	5,259	9,714	11,883
Net earned premiums	4,797	5,556	9,583	10,939
Marine
Segment Reporting Information [Line Items]
Gross written premiums	97,774	97,033	229,277	223,700
Net written premiums	73,844	71,574	177,733	172,219
Net earned premiums	79,611	75,281	157,148	151,935
Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	142,253	110,919	274,443	210,224
Net written premiums	84,697	68,224	155,990	129,632
Net earned premiums	71,610	60,934	126,439	123,930
Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	38,687	45,616	71,277	89,789
Net written premiums	24,822	25,207	42,716	52,471
Net earned premiums	$ 22,556	$ 25,256	$ 42,668	$ 49,675