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Short-Term Debt - Additional Information (Detail) (USD $)
 3 Months Ended 6 Months Ended
Jun. 30, 2014
Jun. 30, 2014
Dec. 31, 2013
Short-term Debt [Line Items]      
Securities pledged as collateral $ 1,030,000,000 $ 1,030,000,000  
Average balance of short-term debt 1,300,000,000 1,200,000,000  
Accrued interest payable on short-term debt 1,400,000 1,400,000 1,000,000
Committed line of credit with financial institutions 10,000,000 10,000,000  
Committed line of credit with financial institutions, outstanding 0 0 0
Residential Loans
      
Short-term Debt [Line Items]      
Loan pledged as collateral 954,000,000 954,000,000  
Commercial Loans
      
Short-term Debt [Line Items]      
Loan pledged as collateral $ 17,000,000 $ 17,000,000  
Credit Agreement
      
Short-term Debt [Line Items]      
Number of bank counterparties 15 15