Employee Benefit Plans - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Age Services	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee pretax contribution percentage	100.00%
Employees compensation	50.00%
Corporation's matching contribution and related expenses	$ 802	$ 652	$ 508
Profit sharing contributions	6.00%
Compensation percentage in excess of specified amount	5.70%
Excess of compensation	$ 127
Recognized profit sharing expense	1,755	1,276	868
Obligations of other liabilities	5,349	4,958
Expenses related employee benefit plans	648	550	608
Obligations under survivor benefit plan	1,264	1,175
Other liabilities related to survivor benefit plan	$ 89	33	$ 155
Employee retirement health care benefits plan | Age	60
Minimum service period for participation in benefit plan | Services	30
Accumulated benefit obligation	$ 2,740	$ 3,409
Amortized from accumulated other comprehensive income into net periodic benefit cost	$ 84
Weighted average discount rate net periodic benefit cost	3.49%	3.67%	3.42%
Weighted average rate accrued benefit obligations	3.13%	3.49%	3.67%
Expected costs of benefit	5.00%
Percent rate of increase in the health care trend	1.00%
Increase in health care trend rates	$ 267
Increase service and interest costs	$ 19
Percent rate of decrease in the health care trend	1.00%
Decrease in health care trend rates	$ 218
Decrease service and interest costs	$ 15
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee contribution percentage	1.00%
Total contributed by employee percentage	1.00%
Qualifying dependents age group | Age	60
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee contribution percentage	3.00%
Total contributed by employee percentage	2.00%
Qualifying dependents age group | Age	65