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Investment Securities - Non-Marketable Securities (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares	Apr. 26, 2018	Dec. 31, 2017 USD ($)
Schedule of Cost-method Investments [Line Items]
Federal Home Loan Bank stock, par value (usd per share) | $ / shares	$ 100
Investments in low income housing tax credit funds	$ 5,000,000		$ 2,100,000
Low income housing tax credits and other tax benefits	110,000
Low income housing amortization expense	94,000
Unfunded commitments for low income housing tax credit funds	$ 3,200,000
Visa Inc. | Visa Inc. Class B common stock
Schedule of Cost-method Investments [Line Items]
Conversion rate on common stock	1.6483
Visa Inc. Class B common stock
Schedule of Cost-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	3,300,000		3,200,000
Subsequent event
Schedule of Cost-method Investments [Line Items]
Federal Home Loan Bank, dividend rate percentage		7.00%
Other assets
Schedule of Cost-method Investments [Line Items]
Federal Home Loan Bank stock	$ 11,100,000		$ 11,100,000

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