Employee Benefit Plans - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) Age Services	Dec. 31, 2015 USD ($)	Dec. 31, 2014 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	$ 652	$ 508	$ 620
Profit sharing contributions	6.00%
Compensation percentage in excess of specified amount	5.70%
Excess of compensation	$ 119
Recognized profit sharing expense	1,276	868	1,044
Obligations of other liabilities	4,958	4,666
Expenses related employee benefit plans	550	608	454
Obligations under survivor benefit plan	1,175	1,142
Other liabilities related to survivor benefit plan	$ 33	155	$ 55
Employee retirement health care benefits plan | Age	60
Minimum service period for participation in benefit plan | Services	30
Accumulated benefit obligation	$ 3,409	$ 3,066
Amortized from accumulated other comprehensive income into net periodic benefit cost	$ 191
Weighted average discount rate net periodic benefit cost	3.67%	3.42%	4.31%
Weighted average rate accrued benefit obligations	3.49%	3.67%	3.42%
Expected costs of benefit	5.00%
Percent rate of increase in the health care trend	1.00%
Increase in health care trend rates	$ 309
Increase service and interest costs	$ 23
Percent rate of decrease in the health care trend	1.00%
Decrease in health care trend rates	$ 276
Decrease service and interest costs	$ 21
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	1.00%
Total contributed by employee percentage	4.00%
Qualifying dependents age group | Age	65