Employee Benefit Plans - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019 USD ($) service age	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee pretax contribution percentage	100.00%
Employee contribution	1.00%
Employee contribution percentage	3.00%
Employees compensation	50.00%
Total contributed by employee percentage	2.00%
Corporation's matching contribution and related expenses	$ 906,000	$ 810,000	$ 802,000
Profit sharing contributions	6.00%
Compensation percentage in excess of specified amount	5.70%
Excess of compensation	$ 133,000
Recognized profit sharing expense	1,949,000	1,530,000	1,755,000
Obligations of other liabilities	6,333,000	5,867,000
Expenses related employee benefit plans	724,000	776,000	648,000
Obligations under survivor benefit plan	1,330,000	1,330,000
Other liabilities related to survivor benefit plan	$ (1,000)	66,000	$ 89,000
Employee retirement health care benefits plan | age	60
Minimum service period for participation in benefit plan | service	30
Accumulated benefit obligation	$ 2,030,000	$ 2,427,000
Amortized from accumulated other comprehensive income into net periodic benefit cost	$ 0
Weighted average discount rate net periodic benefit cost	3.78%	3.13%	3.49%
Accrued benefit obligations	2.59%	3.78%	3.13%
Expected costs of benefit	5.00%
Increase in health care trend rates	$ 155,000
Increase service and interest costs	10,000
Decrease in health care trend rates	141,000
Decrease service and interest costs	$ 9,000
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Qualifying dependents age group | age	60
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Qualifying dependents age group | age	65