Employee Benefit Plans - Additional Information (Detail)	12 Months Ended
Dec. 31, 2020 USD ($) Employee shares	Dec. 31, 2019 USD ($) Employee shares	Dec. 31, 2018 USD ($) Employee shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum percentage of eligible compensation on a pre-tax basis for eligible employees to save	1.00%
Employee contribution matching percentage	50.00%
Percentage of total eligible compensation, contribution by Bank, maximum limit	4.00%
Vesting percentage in participant's deferral contributions and employer matching contributions	100.00%
Vesting period	6 years
Employer matching contribution and the discretionary contribution \| $	$ 1,000,000.0	$ 1,200,000	$ 1,100,000
Restricted stock awards, shares issued	37,382	38,100	33,000
Awards grants originated in 2016, 2017 and 2018 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of employees, received the stock \| Employee		3
Four employees received shares from awards granted due to retirement		14,300
Awards grants originated in 2019, 2018 and 2017 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of employees, received the stock \| Employee	1
Number of shares vested	1,050
Awards grants originated in 2017, 2018 and 2019 and 2020 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of employees, received the stock \| Employee	8
Number of shares vested	10,890
Restricted Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Restricted stock awards, shares issued	37,382	38,100	33,000
Stock issued to number of employees \| Employee	92	94	80
Shares vest under the plan	100.00%	100.00%	100.00%
Shares forfeited due to employee termination	2,175	3,220	2,620
Number of shares vested	35,431	40,370	28,790
Number of employees, received the stock \| Employee	57	63	56
Number of shares vested	10,890	14,300
Compensation expense \| $	$ 1,000,000.0	$ 1,100	$ 745,000
Unrecognized compensation cost related to the nonvested portion of restricted stock awards under the plan \| $	$ 1,300,000
Unrecognized compensation cost related to the nonvested portion of restricted stock awards under the plan, period for recognition	3 years
Restricted Stock Awards [Member] \| 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares vested			28,790
Restricted Stock Awards [Member] \| 2019 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares vested	1,600
Number of employees, received the stock \| Employee	3
Restricted Stock Awards [Member] \| 2015 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares vested		26,070
Restricted Stock Awards [Member] \| 2017 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares vested	21,340
Number of employees, received the stock \| Employee	53
Restricted Stock Awards [Member] \| Immediate Vesting [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of employees, received the stock \| Employee	1
Number of shares vested	551,000

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Number of awards granted to four employees on retirement.

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Share based compensation arrangement by share based payment award number of employee share issued.

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Share based compensation arrangement by share based payment award number of employees.

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Amount of discretionary contributions made by an employer to a defined contribution plan.

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Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

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Percentage of employer's matching contributions to a defined contribution plan that vests in a given year.

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Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Amount of cost to be recognized for nonvested award under share-based payment arrangement. Excludes share and unit options.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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Total number of shares issued during the period, including shares forfeited, as a result of Restricted Stock Awards.

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