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Loan Servicing - Additional Information (Details) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Transfers And Servicing [Abstract]      
Escrow balances maintained in connection with serviced loans $ 1,400,000 $ 961,000  
Mortgage servicing rights, valuation allowance $ 0 $ 0 $ 0