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Securities - Realized Gains and Losses (Narrative) (Details)
6 Months Ended 12 Months Ended
Jun. 30, 2018
USD ($)
security
Dec. 31, 2016
USD ($)
Dec. 31, 2017
USD ($)
Schedule of Investments [Line Items]      
Available-for-sale securities, impairment on securities   $ 0  
Realized investment gains $ 16,000    
Available-for-sale securities pledged as collateral 3,800,000   $ 20,800,000
Residential mortgage-backed securities | 3 Securities Sold      
Schedule of Investments [Line Items]      
Realized investment gains $ 71,000    
Number of securities sold | security 3    
Available-for-sale securities, book value $ 1,200,000    
Residential mortgage-backed securities | 2 Securities Sold      
Schedule of Investments [Line Items]      
Realized investment gains $ (58,000)    
Number of securities sold | security 2    
Available-for-sale securities, book value $ 1,600,000    
Asset-backed securities      
Schedule of Investments [Line Items]      
Realized investment gains $ 3,000    
Number of securities sold | security 2    
Available-for-sale securities, book value $ 3,500,000