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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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