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Borrowings - Additional Information (Details)
12 Months Ended
Dec. 31, 2016
USD ($)
Institution
Dec. 31, 2015
USD ($)
Jan. 31, 2017
Jan. 31, 2016
Debt Instrument [Line Items]        
Overnight FHLB advances $ 1,300,000 $ 796,000,000    
FHLB advances interest rates       0.27%
Loans pledged as collateral 2,100,000,000      
Federal Home Loan Bank, maximum borrowing capacity 1,800,000      
Federal Home Loan Bank, additional credit available 129,000,000      
Unsecured debt $ 115,000,000      
Number of financial institutions | Institution 4      
Long-term line of credit $ 0      
Federal Home Loan Bank unused lines of credit 171,800,000      
FHLB, average daily balance of borrowings outstanding 507,000,000 $ 352,700,000    
Minimum        
Debt Instrument [Line Items]        
Unsecured debt 20,000,000      
Maximum        
Debt Instrument [Line Items]        
Unsecured debt $ 25,000,000      
Subsequent Event        
Debt Instrument [Line Items]        
FHLB advances interest rates     0.56%