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Borrowings and Subordinated Debentures (Details Textual) - USD ($)
3 Months Ended 12 Months Ended
Jun. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Unused lines of Credit      
Debt Instrument [Line Items]      
Line of Credit Facility, Amount Outstanding   $ 0  
Line of Credit Facility, Current Borrowing Capacity   180,000,000.0  
Debt Instrument, Unused Borrowing Capacity, Fee   0  
American Financial Exchange      
Debt Instrument [Line Items]      
Line of Credit Facility, Amount Outstanding   0 $ 300,000,000.0
Federal Home Loan Bank of San Francisco      
Debt Instrument [Line Items]      
Line of Credit, Current   5,000,000 1,318,000,000
Loans Pledged as Collateral   5,600,000,000  
Prepayment Of Federal Home Loan Bank Advances $ 750,000,000.0    
Prepayment Penalties Federal Home Loan Bank Advances $ 6,600,000    
Debt, Weighted Average Interest Rate 0.96%    
Federal Home Loan Bank of San Francisco | Secured Debt      
Debt Instrument [Line Items]      
Line of Credit Facility, Borrowing Capacity, Description   3,300,000,000  
Line of Credit, Current   5,000,000 1,318,000,000
Federal Home Loan Bank of San Francisco | Unused lines of Credit      
Debt Instrument [Line Items]      
Line of Credit Facility, Borrowing Capacity, Description   112,000,000.0  
Federal Home Loan Bank of San Francisco | Unsecured Debt      
Debt Instrument [Line Items]      
Line of Credit, Current   0 141,000,000.0
Federal Reserve Bank Advances [Member]      
Debt Instrument [Line Items]      
Line of Credit Facility, Amount Outstanding   0 0
Loans Pledged as Collateral   1,900,000,000  
Federal Reserve Bank Advances [Member] | Secured Debt      
Debt Instrument [Line Items]      
Line of Credit Facility, Borrowing Capacity, Description   1,400,000,000  
Maturity Overnight | Federal Home Loan Bank of San Francisco      
Debt Instrument [Line Items]      
Line of Credit, Current   $ 0 $ 1,318,000,000