Mortgage Loan Portfolio as Collectively or Individually Evaluated for Impairment (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Financing Receivable, Recorded Investment [Line Items]    
Mortgage loans collectively evaluated for impairment $ 563,214 $ 604,772
Mortgage loans individually evaluated for impairment      
Allowance for potential future losses (2,442) (2,849)
Carrying value $ 560,772 $ 601,923