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Goodwill and Intangible Assets (Additional Information) (Details)
1 Months Ended 3 Months Ended 9 Months Ended 59 Months Ended
Jan. 02, 2020
USD ($)
Jan. 31, 2020
USD ($)
Mar. 31, 2016
USD ($)
Mar. 31, 2015
USD ($)
Sep. 30, 2020
USD ($)
Sep. 30, 2019
USD ($)
Sep. 30, 2020
USD ($)
Sep. 30, 2019
USD ($)
Jan. 31, 2020
acquisition
Dec. 31, 2019
USD ($)
Goodwill [Line Items]                    
Number of banks acquired | acquisition                 3  
Goodwill recognized in the acquisition         $ 35,554,000   $ 35,554,000     $ 9,387,000
Goodwill impairment             0      
Amortization of core deposit intangibles             576,000 $ 117,000    
Serviced mortgage loans, unpaid principal balance         211,000,000.0   211,000,000.0     $ 9,000,000.0
Lotus Bank                    
Goodwill [Line Items]                    
Goodwill acquired       $ 4,600,000            
Bank of Michigan                    
Goodwill [Line Items]                    
Goodwill acquired     $ 4,800,000              
Ann Arbor Bancorp, Inc                    
Goodwill [Line Items]                    
Goodwill acquired $ 26,200,000 $ 26,200,000                
Goodwill recognized in the acquisition $ 26,167,000                  
Core Deposits                    
Goodwill [Line Items]                    
Amortization of core deposit intangibles         $ 191,000 $ 29,000 $ 576,000 $ 117,000