Cash Value of Life Insurance - Summary of Activity in Balance of Cash Value of Life Insurance (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Policies Employees Company	Dec. 31, 2016 USD ($) Policies Employees Company	Dec. 31, 2015 USD ($)
Increase (Decrease) in Insurance Liabilities [Abstract]
Beginning balance	$ 95,912	$ 94,560
Increase in cash value of life insurance	2,685	2,717	$ 2,786
Death benefit receivable in excess of cash value	108	238
Death benefit receivable	(922)	(1,603)
Ending balance	97,783	95,912	$ 94,560
End of period death benefit	$ 165,587	$ 165,669
Number of policies owned | Policies	182	185
Insurance companies used | Company	14	14
Current and former employees and directors covered | Employees	57	58