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Investment Securities - Non-Marketable Securities (Details)		3 Months Ended		12 Months Ended
	Nov. 27, 2018	Mar. 31, 2019 USD ($) $ / shares shares	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) shares	Apr. 25, 2019
Schedule of Equity Method Investments [Line Items]
Federal Home Loan Bank stock, par value (usd per share) | $ / shares		$ 100
Conversion rate on common stock	2
Investments in low income housing tax credit funds		$ 4,500,000		$ 4,600,000
Low income housing tax credits and other tax benefits		154,000
Low income housing amortization expense		130,000
Unfunded commitments for low income housing tax credit funds		$ 3,100,000
Visa Inc. Class B common stock
Schedule of Equity Method Investments [Line Items]
Number of shares of securities carried at cost (in shares) | shares		10,439		10,439,000
Carrying value of securities carried at cost		$ 0		$ 0
Fair value of Class B common stock		$ 2,700,000		$ 2,200,000
Visa Inc. | Visa Inc. Class B common stock
Schedule of Equity Method Investments [Line Items]
Conversion rate on common stock		1.6298		1.6298
Other assets
Schedule of Equity Method Investments [Line Items]
Federal Home Loan Bank stock		$ 11,100,000		$ 11,100,000
Subsequent Event
Schedule of Equity Method Investments [Line Items]
Federal Home Loan Bank, dividend rate percentage					7.00%
Low-Income Housing Tax Credit Investment
Schedule of Equity Method Investments [Line Items]
Impairment losses		$ 0	$ 0

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