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Investment Securities - Non-Marketable Securities (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($)	Jul. 26, 2018
Schedule of Equity Method Investments [Line Items]
Federal Home Loan Bank stock, par value (usd per share) | $ / shares	$ 100
Investments in low income housing tax credit funds	$ 4,900,000	$ 2,100,000
Low income housing tax credits and other tax benefits	282,000
Low income housing amortization expense	237,000
Unfunded commitments for low income housing tax credit funds	$ 3,200,000
Visa Inc. Class B common stock
Schedule of Equity Method Investments [Line Items]
Number of shares of securities carried at cost | shares	16,939
Carrying value of securities carried at cost	$ 0
Fair value of Class B common stock	$ 3,700,000	$ 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
Subsequent Event
Schedule of Equity Method Investments [Line Items]
Federal Home Loan Bank, dividend rate percentage			7.00%

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