Additional Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Gross written premiums	$ 333,091	$ 312,076	$ 332,128	$ 393,222	$ 321,587	$ 298,742	$ 322,987	$ 343,149	$ 1,370,517	$ 1,286,465	$ 1,108,216
Net written premiums	223,445	196,556	198,469	269,452	212,312	188,046	190,252	243,045	887,922	833,655	753,798
Net earned premiums	219,902	213,895	205,814	202,328	201,566	201,262	196,017	183,119	841,939	781,964	691,645
Marine
Segment Reporting Information [Line Items]
Gross written premiums									352,868	394,518	396,062
Net written premiums									254,464	276,810	307,848
Net earned premiums									267,966	279,079	314,677
Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									829,609	717,769	560,425
Net written premiums									505,276	433,992	350,007
Net earned premiums									447,202	385,733	287,200
Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									188,040	174,178	151,729
Net written premiums									128,182	122,853	95,943
Net earned premiums									126,771	117,152	89,768
Insurance
Segment Reporting Information [Line Items]
Gross written premiums									1,002,275	921,325	788,419
Net written premiums									680,008	622,956	542,391
Net earned premiums									639,338	571,439	472,463
Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums									171,822	200,095	228,500
Net written premiums									119,837	133,210	170,642
Net earned premiums									129,276	142,181	169,018
Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									700,087	590,741	445,287
Net written premiums									462,942	390,168	293,758
Net earned premiums									409,480	332,782	231,297
Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									130,366	130,489	114,632
Net written premiums									97,229	99,578	77,991
Net earned premiums									100,582	96,476	72,148
Lloyd's
Segment Reporting Information [Line Items]
Gross written premiums									368,242	365,140	319,797
Net written premiums									207,914	210,699	211,407
Net earned premiums									202,601	210,525	219,182
Lloyd's | Marine
Segment Reporting Information [Line Items]
Gross written premiums									181,046	194,423	167,562
Net written premiums									134,627	143,600	137,206
Net earned premiums									138,690	136,898	145,659
Lloyd's | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									129,522	127,028	115,138
Net written premiums									42,334	43,824	56,249
Net earned premiums									37,722	52,951	55,903
Lloyd's | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									57,674	43,689	37,097
Net written premiums									30,953	23,275	17,952
Net earned premiums									$ 26,189	$ 20,676	$ 17,620