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Income Taxes - Additional Information (Detail) (USD $)
3 Months Ended9 Months Ended
Sep. 30, 2011
Sep. 30, 2010
Sep. 30, 2011
Sep. 30, 2010
Dec. 31, 2010
Income Taxes [Line Items]     
Undistributed earnings of non-U.S. subsidiaries  $ 61,600,000  
Taxes payable on undistributed earnings1,600,000 1,600,000  
Deferred Tax Assets: State and Local1,800,000 1,800,000 2,200,000
State and local net operating loss carry-forwards1,300,000 1,300,000 1,400,000
Income tax (benefit) expense$ 4,476,000$ 7,091,000$ 5,015,000$ 21,659,000 
Effective income tax rate24.40%30.40%24.40%29.30% 
Syndicate 1221 | Minimum | UNITED KINGDOM
     
Income Taxes [Line Items]     
Foreign effective income tax rate  35.00%  
UNITED KINGDOM
     
Income Taxes [Line Items]     
Foreign effective income tax rate  35.00%  
Corporate tax rate  28.00%  
UNITED KINGDOM | Expected Future Tax Rate
     
Income Taxes [Line Items]     
Corporate tax rate  26.00%  
Net investment income
     
Income Taxes [Line Items]     
Effective income tax rate28.60%28.40%28.60%26.70% 
Maximum | State and Local Jurisdiction
     
Income Taxes [Line Items]     
State and local tax carry-forwards expiration year  2030  
Minimum | State and Local Jurisdiction
     
Income Taxes [Line Items]     
State and local tax carry-forwards expiration year  2024