Directors Retirement Plan - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Minimum period of service for retirement	5 years
Maximum payment period for benefits	10 years
Defined benefit plan, expected amortization, next fiscal year	$ 0
Director's Retirement Plan
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Discount rate of directors retirement plan	2.21%	2.89%	3.97%
Plan assets of director's retirement plan	$ 0
Contribution to the director's retirement plan	37,000
Minimum
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Remuneration rate for board of directors	5,000
Maximum
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Remuneration rate for board of directors	$ 17,500