Additional Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Gross written premiums	$ 321,587	$ 298,742	$ 322,987	$ 343,149	$ 277,901	$ 255,318	$ 278,714	$ 296,283	$ 1,286,465	$ 1,108,216	$ 987,201
Net written premiums	212,312	188,046	190,252	243,045	202,002	175,357	183,363	193,076	833,655	753,798	653,938
Net earned premiums	201,566	201,262	196,017	183,119	191,757	173,633	173,777	152,478	781,964	691,645	659,931
Marine
Segment Reporting Information [Line Items]
Gross written premiums									394,518	396,062	405,784
Net written premiums									276,810	307,848	300,399
Net earned premiums									279,079	314,677	305,071
Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									717,769	560,425	407,450
Net written premiums									433,992	350,007	251,894
Net earned premiums									385,733	287,200	250,593
Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									174,178	151,729	173,967
Net written premiums									122,853	95,943	101,645
Net earned premiums									117,152	89,768	104,267
Insurance Companies
Segment Reporting Information [Line Items]
Gross written premiums									921,325	788,419	665,505
Net written premiums									622,956	542,391	429,355
Net earned premiums									571,439	472,463	438,851
Insurance Companies | Marine
Segment Reporting Information [Line Items]
Gross written premiums									200,095	228,500	223,061
Net written premiums									133,210	170,642	151,059
Net earned premiums									142,181	169,018	155,846
Insurance Companies | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									590,741	445,287	312,651
Net written premiums									390,168	293,758	197,845
Net earned premiums									332,782	231,297	200,741
Insurance Companies | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									130,489	114,632	129,793
Net written premiums									99,578	77,991	80,451
Net earned premiums									96,476	72,148	82,264
Lloyd's Operations
Segment Reporting Information [Line Items]
Gross written premiums									365,140	319,797	321,696
Net written premiums									210,699	211,407	224,583
Net earned premiums									210,525	219,182	221,080
Lloyd's Operations | Marine
Segment Reporting Information [Line Items]
Gross written premiums									194,423	167,562	182,723
Net written premiums									143,600	137,206	149,340
Net earned premiums									136,898	145,659	149,225
Lloyd's Operations | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									127,028	115,138	94,799
Net written premiums									43,824	56,249	54,049
Net earned premiums									52,951	55,903	49,852
Lloyd's Operations | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									43,689	37,097	44,174
Net written premiums									23,275	17,952	21,194
Net earned premiums									$ 20,676	$ 17,620	$ 22,003