Income Taxes - Components of Pre-tax Income and Provision for Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Provision for income taxes:
Currently payable	$ 5,736	$ 5,621	$ 4,778
Deferred income tax expense (benefit)	(1,775)	(114)	125
Provision for income taxes	3,961	5,507	4,903
Expected tax expense, at the U.S. federal statutory income tax rate of 35%	6,007	5,588	4,908
Changes in federal tax rate, net	(1,861)
State income taxes	235	388	387
Domestic production activities deduction	(505)	(495)	(415)
Other, net	85	26	23
Provision for income taxes	$ 3,961	$ 5,507	$ 4,903

Amount of current income tax expense (benefit) pertaining to taxable income (loss) from continuing operations.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations, attributable to increase (decrease) in the income tax rates.

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