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Acquisition Activity (Details) - USD ($)
Oct. 01, 2019
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Business Acquisition [Line Items]        
Goodwill   $ 0 $ 739,023,000 $ 739,023,000
Independent Bank        
Business Acquisition [Line Items]        
Loans under management acquired $ 306,000,000.0      
Total consideration paid 4,711,000      
Goodwill 1,539,000      
Goodwill deductible for tax purposes 0      
Independent Bank | Customer relationships        
Business Acquisition [Line Items]        
Customer relationship intangible $ 3,172,000      
Estimated useful life (in years) 13 years