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Investments - Additional Information (Details)
$ in Thousands
3 Months Ended
Jun. 30, 2018
USD ($)
security
Dec. 31, 2018
USD ($)
security
Dec. 31, 2017
USD ($)
security
Investment Holdings [Line Items]      
Amortized Cost   $ 182,325 $ 247,607
Par value of mortgage-backed securities pledged to FRB   147,000 218,000
Mortgage-backed securities      
Investment Holdings [Line Items]      
Amortized Cost   $ 159,937 $ 227,607
Number Of Mortgage-Backed Securities Sold | security 9    
Proceeds from sale of securities $ 41,000    
Recognized loss from sale of securities $ 2,000    
Number of mortgage-backed securities with unrealized losses | security   74 62
Number of mortgage-backed securities | security   86 92
Utah Housing Corporation bonds      
Investment Holdings [Line Items]      
Amortized Cost   $ 22,388 $ 20,000
Ginnie Mae | Mortgage-backed securities      
Investment Holdings [Line Items]      
Amortized Cost   $ 64,000  
Number of mortgage-backed securities with unrealized losses | security   34 31
Fannie Mae | Mortgage-backed securities      
Investment Holdings [Line Items]      
Amortized Cost   $ 46,000  
Freddie Mac | Mortgage-backed securities      
Investment Holdings [Line Items]      
Amortized Cost   50,000  
Aa3 Rating | Utah Housing Corporation bonds      
Investment Holdings [Line Items]      
Amortized Cost   $ 22,000 $ 0