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Schedule of Liability for Unpaid Losses and Loss Adjustment Expenses (Parenthetical) (Detail) (Chaucer Business, USD $)
In Millions, unless otherwise specified
Dec. 31, 2011
Chaucer Business
 
Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses [Line Items]  
Purchase of Chaucer, reinsurance recoverable on unpaid losses $ 669.6