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Investment Securities - Non-Marketable Securities (Details) |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | ---
Oct. 25, 2018 USD ($) shares | Oct. 22, 2018 | Sep. 30, 2018 USD ($) $ / shares shares | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) $ / shares shares | Sep. 30, 2017 USD ($) | Dec. 31, 2017 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Federal Home Loan Bank stock, par value (usd per share) | $ / shares |  |  | $ 100 |  | $ 100 |  |
Investments in low income housing tax credit funds |  |  | $ 4,800,000 |  | $ 4,800,000 |  | $ 2,100,000
Low income housing tax credits and other tax benefits |  |  | 173,000 |  | 454,000 |  |
Low income housing amortization expense |  |  | 144,000 |  | 381,000 |  |
Unfunded commitments for low income housing tax credit funds |  |  | $ 3,200,000 |  | $ 3,200,000 |  |
Visa Inc. Class B common stock |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Number of shares of securities carried at cost (in shares) | shares |  |  | 16,939 |  | 16,939 |  |
Carrying value of securities carried at cost |  |  | $ 0 |  | $ 0 |  |
Fair value of Class B common stock |  |  | 4,100,000 |  | $ 4,100,000 |  | $ 3,200,000
Visa Inc. | Visa Inc. Class B common stock |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Conversion rate on common stock |  |  |  |  | 1.6298 |  | 1.6483
Other assets |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Federal Home Loan Bank stock |  |  | 11,100,000 |  | $ 11,100,000 |  | $ 11,100,000
Low-Income Housing Tax Credit Investment |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Impairment losses |  |  | $ 0 | $ 0 | $ 0 | $ 0 |
Subsequent Event |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Federal Home Loan Bank, dividend rate percentage | 7.00% |  |  |  |  |  |
Conversion rate on common stock |  | 2 |  |  |  |  |
Subsequent Event | Visa Inc. Class B common stock |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Number of shares sold during the period (in shares) | shares | 6,500 |  |  |  |  |  |
Scenario, Forecast | Visa Inc. Class B common stock |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Estimated net pre-tax gain from sale of Visa class B common stock | $ 955,000 |  |  |  |  |  |

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