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Short-Term Debt - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2015
Dec. 31, 2014
Short-term Debt [Line Items]      
Debt reclassified from long-term debt to short-term debt $ 26,000,000    
Amount borrowed from FHLBC 100,000,000 $ 100,000,000  
Securities pledged as collateral     $ 762,000,000
Average balance of short-term debt 1,690,000,000 1,560,000,000  
Accrued interest payable on short-term debt 2,000,000 2,000,000 2,000,000
Committed line of credit 10,000,000 10,000,000  
Fair value of mortgage backed securities securing loan (in excess) 11,000,000 11,000,000  
Committed line of credit with financial institutions, outstanding 0 0 0
Residential Loans      
Short-term Debt [Line Items]      
Loan pledged as collateral 1,300,000,000 1,300,000,000 1,220,000,000
Commercial Loans      
Short-term Debt [Line Items]      
Loan pledged as collateral 185,000,000 185,000,000 $ 161,000,000
Residential      
Short-term Debt [Line Items]      
Securities pledged as collateral $ 587,000,000 $ 587,000,000