Additional Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Segment Reporting Information [Line Items]
Gross written premiums	$ 322,987	$ 278,714	$ 666,136	$ 574,997
Net written premiums	190,252	183,363	433,297	376,439
Net earned premiums	196,017	173,777	379,136	326,255
Marine
Segment Reporting Information [Line Items]
Gross written premiums	96,378	97,774	220,573	229,277
Net written premiums	64,811	73,844	156,201	177,733
Net earned premiums	68,462	79,611	138,346	157,148
Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	180,160	142,253	359,820	274,443
Net written premiums	93,014	84,697	216,434	155,990
Net earned premiums	98,792	71,610	186,317	126,439
Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	46,449	38,687	85,743	71,277
Net written premiums	32,427	24,822	60,662	42,716
Net earned premiums	28,763	22,556	54,473	42,668
Insurance Companies
Segment Reporting Information [Line Items]
Gross written premiums	214,064	186,767	462,402	393,543
Net written premiums	134,065	123,204	315,315	253,944
Net earned premiums	141,577	114,987	273,125	213,807
Insurance Companies | Marine
Segment Reporting Information [Line Items]
Gross written premiums	49,896	58,323	111,761	128,671
Net written premiums	31,786	41,802	74,651	96,020
Net earned premiums	35,535	41,877	70,810	82,436
Insurance Companies | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	130,627	100,131	286,546	213,019
Net written premiums	76,842	62,015	191,374	124,922
Net earned premiums	82,175	55,351	156,543	98,286
Insurance Companies | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	33,541	28,313	64,095	51,853
Net written premiums	25,437	19,387	49,290	33,002
Net earned premiums	23,867	17,759	45,772	33,085
Lloyd's Operations
Segment Reporting Information [Line Items]
Gross written premiums	108,923	91,947	203,734	181,454
Net written premiums	56,187	60,159	117,982	122,495
Net earned premiums	54,440	58,790	106,011	112,448
Lloyd's Operations | Marine
Segment Reporting Information [Line Items]
Gross written premiums	46,482	39,451	108,812	100,606
Net written premiums	33,025	32,042	81,550	81,713
Net earned premiums	32,927	37,734	67,536	74,712
Lloyd's Operations | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	49,533	42,122	73,274	61,424
Net written premiums	16,172	22,682	25,060	31,068
Net earned premiums	16,617	16,259	29,774	28,153
Lloyd's Operations | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	12,908	10,374	21,648	19,424
Net written premiums	6,990	5,435	11,372	9,714
Net earned premiums	$ 4,896	$ 4,797	$ 8,701	$ 9,583