Directors Retirement Plan - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Minimum period of service for retirement	5 years
Maximum payment period for benefits	10 years
Accumulated other comprehensive income expected to be recognized as a component of net periodic benefit cost	$ 0
Director's Retirement Plan
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Discount rate of directors retirement plan	3.30%	3.68%	3.87%
Plan assets of director's retirement plan	$ 0
Contribution to the director's retirement plan	75,000
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