Additional Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Gross written premiums	$ 333,299	$ 327,469	$ 348,795	$ 422,790	$ 333,091	$ 312,076	$ 332,128	$ 393,222	$ 1,432,353	$ 1,370,517	$ 1,286,465
Net written premiums	228,007	228,417	231,864	311,850	223,445	196,556	198,469	269,452	1,000,138	887,922	833,655
Net earned premiums	234,172	245,367	231,084	225,272	219,902	213,895	205,814	202,328	935,895	841,939	781,964
Marine
Segment Reporting Information [Line Items]
Gross written premiums									365,470	352,868	394,518
Net written premiums									267,944	254,464	276,810
Net earned premiums									264,674	267,966	279,079
Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									881,075	829,609	717,769
Net written premiums									610,761	505,276	433,992
Net earned premiums									547,547	447,202	385,733
Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									185,808	188,040	174,178
Net written premiums									121,433	128,182	122,853
Net earned premiums									123,674	126,771	117,152
Insurance Companies
Segment Reporting Information [Line Items]
Gross written premiums									1,045,454	1,002,275	921,325
Net written premiums									752,773	680,008	622,956
Net earned premiums									704,574	639,338	571,439
Insurance Companies | Marine
Segment Reporting Information [Line Items]
Gross written premiums									177,363	171,822	200,095
Net written premiums									123,617	119,837	133,210
Net earned premiums									123,203	129,276	142,181
Insurance Companies | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									755,059	700,087	590,741
Net written premiums									554,844	462,942	390,168
Net earned premiums									496,209	409,480	332,782
Insurance Companies | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									113,032	130,366	130,489
Net written premiums									74,312	97,229	99,578
Net earned premiums									85,162	100,582	96,476
Lloyd's
Segment Reporting Information [Line Items]
Gross written premiums									386,899	368,242	365,140
Net written premiums									247,365	207,914	210,699
Net earned premiums									231,321	202,601	210,525
Lloyd's | Marine
Segment Reporting Information [Line Items]
Gross written premiums									188,107	181,046	194,423
Net written premiums									144,327	134,627	143,600
Net earned premiums									141,471	138,690	136,898
Lloyd's | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									126,016	129,522	127,028
Net written premiums									55,917	42,334	43,824
Net earned premiums									51,338	37,722	52,951
Lloyd's | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									72,776	57,674	43,689
Net written premiums									47,121	30,953	23,275
Net earned premiums									$ 38,512	$ 26,189	$ 20,676