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Investment Securities - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) investment_security	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) investment_security	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Investment Securities [Line Items]
Proceeds from sale of available for sale securities			$ 229,000	$ 125,247,000
Loss on sale of investments	$ 0	$ 228,000	0	228,000
Gain on sale of investment securities	7,000	$ 335,000	7,000	$ 335,000
Securities pledged as collateral	460,317,000		460,317,000		$ 466,321,000
Residential real estate mortgage-backed securities	856,092,000,000		$ 856,092,000,000
Life of mortgage-backed securities			2 years 9 months 3 days
Impairment losses recognized	0		$ 0
Allowance for credit losses	0		0		$ 0
Obligations of U.S. government agencies
Investment Securities [Line Items]
Allowance for credit losses	$ 0		$ 0
Number of available for sale securities in unrealized loss position (in investment securities) | investment_security	13		13
Percentage of aggregate depreciation in unrealized losses	0.15%		0.15%
Obligations of states and political subdivisions
Investment Securities [Line Items]
Allowance for credit losses	$ 0		$ 0
Number of available for sale securities in unrealized loss position (in investment securities) | investment_security	1		1
Percentage of aggregate depreciation in unrealized losses	1.33%		1.33%
Asset backed securities
Investment Securities [Line Items]
Allowance for credit losses	$ 0		$ 0
Number of available for sale securities in unrealized loss position (in investment securities) | investment_security	9		9
Percentage of aggregate depreciation in unrealized losses	2.41%		2.41%

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Investment Securities [Line Items]

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Life of mortgage backed securities.

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Percentage of aggregate depreciation in unrealized losses.

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Securities Pledged as Collateral

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Amount of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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The difference between the carrying value and the sale price of an investment. A gain would be recognized when the sale price of the investment is greater than the carrying value of the investment. This element refers to the Gain included in earnings and not to the cash proceeds of the sale.

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The difference between the carrying value and the sale price of an investment. A loss would be recognized when the sale price of the investment is less than the carrying value of the investment. This element refers to the Loss included in earnings and not to the cash proceeds of the sale.

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The cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (a component of shareholders' equity).

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Amount of cash inflow from sale of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Debt securities issued by a United States government agency such as Government National Mortgage Association (Ginnie Mae), Federal Home Loan Mortgage Corporation (Freddie Mac), and Federal National Mortgage Association (Fannie Mae), which are short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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