Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting Information [Line Items]
Gross written premiums	$ 277,901	$ 255,318	$ 278,714	$ 296,283	$ 229,850	$ 233,638	$ 253,568	$ 270,145	$ 1,108,216	$ 987,201	$ 1,044,918
Net written premiums	202,002	175,357	183,363	193,076	141,809	157,807	165,005	189,317	753,798	653,938	701,255
Net earned premiums	191,757	173,633	173,777	152,478	166,158	168,233	161,471	164,069	691,645	659,931	683,363
Marine
Segment Reporting Information [Line Items]
Gross written premiums									396,062	405,784	433,397
Net written premiums									307,848	300,399	327,442
Net earned premiums									314,677	305,071	300,492
Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									560,425	407,450	430,436
Net written premiums									350,007	251,894	272,331
Net earned premiums									287,200	250,593	285,473
Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									151,729	173,967	181,085
Net written premiums									95,943	101,645	101,482
Net earned premiums									89,768	104,267	97,398
Insurance Companies
Segment Reporting Information [Line Items]
Gross written premiums									788,419	665,505	730,776
Net written premiums									542,391	429,355	477,673
Net earned premiums									472,463	438,851	479,121
Insurance Companies | Marine
Segment Reporting Information [Line Items]
Gross written premiums									228,500	223,061	241,438
Net written premiums									170,642	151,059	171,289
Net earned premiums									169,018	155,846	157,534
Insurance Companies | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									445,287	312,651	352,285
Net written premiums									293,758	197,845	227,234
Net earned premiums									231,297	200,741	246,143
Insurance Companies | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									114,632	129,793	137,053
Net written premiums									77,991	80,451	79,150
Net earned premiums									72,148	82,264	75,444
Lloyd's Operations
Segment Reporting Information [Line Items]
Gross written premiums									319,797	321,696	314,142
Net written premiums									211,407	224,583	223,582
Net earned premiums									219,182	221,080	204,242
Lloyd's Operations | Marine
Segment Reporting Information [Line Items]
Gross written premiums									167,562	182,723	191,959
Net written premiums									137,206	149,340	156,153
Net earned premiums									145,659	149,225	142,958
Lloyd's Operations | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									115,138	94,799	78,151
Net written premiums									56,249	54,049	45,097
Net earned premiums									55,903	49,852	39,330
Lloyd's Operations | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									37,097	44,174	44,032
Net written premiums									17,952	21,194	22,332
Net earned premiums									$ 17,620	$ 22,003	$ 21,954