Income Taxes - Components of Pre-tax Income and Provision for Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Provision for income taxes:
Currently payable	$ 4,849	$ 5,064	$ 5,736
Deferred income tax expense (benefit)	89	86	(1,775)
Provision for income taxes	4,938	5,150	3,961
Expected tax expense, at the U.S. federal statutory income tax rate of 35% in 2017 and 21% in 2018 and 2019	4,397	4,300	6,007
Changes in federal tax rate, net			(1,861)
State income taxes	742	812	235
Domestic production activities deduction			(505)
Foreign derived intangible income benefit	(201)	(100)
Other, net		38	85
Provision for income taxes	$ 4,938	$ 5,150	$ 3,961