Additional Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting Information [Line Items]
Gross written premiums	$ 332,128	$ 322,987	$ 725,350	$ 666,136
Net written premiums	198,469	190,252	467,921	433,297
Net earned premiums	205,814	196,017	408,142	379,136
Marine
Segment Reporting Information [Line Items]
Gross written premiums	88,970	96,933	193,461	220,573
Net written premiums	61,426	65,278	142,125	156,201
Net earned premiums	66,421	69,807	137,191	138,346
Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	193,312	179,605	437,334	359,820
Net written premiums	102,892	92,547	260,155	216,434
Net earned premiums	108,039	97,447	208,636	186,317
Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	49,846	46,449	94,555	85,743
Net written premiums	34,151	32,427	65,641	60,662
Net earned premiums	31,354	28,763	62,315	54,473
Insurance
Segment Reporting Information [Line Items]
Gross written premiums	228,565	214,064	530,193	462,402
Net written premiums	143,380	134,065	359,699	315,315
Net earned premiums	156,547	141,577	310,878	273,125
Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums	43,077	49,896	93,924	111,761
Net written premiums	27,366	31,786	68,507	74,651
Net earned premiums	31,798	35,535	68,523	70,810
Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	152,095	130,627	371,059	286,546
Net written premiums	90,711	76,842	240,662	191,374
Net earned premiums	99,477	82,175	192,195	156,543
Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	33,393	33,541	65,210	64,095
Net written premiums	25,303	25,437	50,530	49,290
Net earned premiums	25,272	23,867	50,160	45,772
Lloyd's
Segment Reporting Information [Line Items]
Gross written premiums	103,563	108,923	195,157	203,734
Net written premiums	55,089	56,187	108,222	117,982
Net earned premiums	49,267	54,440	97,264	106,011
Lloyd's | Marine
Segment Reporting Information [Line Items]
Gross written premiums	45,893	47,037	99,537	108,812
Net written premiums	34,060	33,492	73,618	81,550
Net earned premiums	34,623	34,272	68,668	67,536
Lloyd's | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	41,217	48,978	66,275	73,274
Net written premiums	12,181	15,705	19,493	25,060
Net earned premiums	8,562	15,272	16,441	29,774
Lloyd's | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	16,453	12,908	29,345	21,648
Net written premiums	8,848	6,990	15,111	11,372
Net earned premiums	$ 6,082	$ 4,896	$ 12,155	$ 8,701