Shareholders' Equity - Summary of Accumulated Other Comprehensive Income (Loss) to Net Income Balances (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income Loss [Line Items]
Balance	$ 249,806	$ 244,245	$ 240,135
Transfer of losses from available for sale to held to maturity		(245)
Other comprehensive income (loss) before reclassifications	2,946	(1,342)	1,036
Accretion of unrealized losses transferred from available for sale to held to maturity recognized in other comprehensive income	129	160	39
Reclassification adjustment for gains realized in income	(368)	(393)	(297)
Total other comprehensive income (loss)	2,707	(1,820)	778
Balance	294,265	249,806	244,245
Accounting Standards Update 2018-02 [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Reclassification from adoption of ASU 2018-02	(352)
Unrealized Gains (Losses) on Securities Available for Sale [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance	(3,130)	(2,492)	(4,315)
Transfer of losses from available for sale to held to maturity		(245)	999
Other comprehensive income (loss) before reclassifications	2,946		916
Reclassification adjustment for gains realized in income	(368)	(393)	(92)
Total other comprehensive income (loss)	2,578	(638)	1,823
Balance	(904)	(3,130)	(2,492)
Unrealized Gains (Losses) on Securities Available for Sale [Member] | Accounting Standards Update 2018-02 [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Reclassification from adoption of ASU 2018-02	(352)
Disproportionate Tax Effect from Securities Available for Sale [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance	(17,110)	(17,110)	(17,110)
Balance	(17,110)	(17,110)	(17,110)
Unrealized Gains (Losses) on Held to Maturity [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance	(800)	(960)
Transfer of losses from available for sale to held to maturity			(999)
Accretion of unrealized losses transferred from available for sale to held to maturity recognized in other comprehensive income	129	160	39
Total other comprehensive income (loss)	129	160	(960)
Balance	(671)	(800)	(960)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance	(21,040)	(19,220)	(19,998)
Total other comprehensive income (loss)	2,707	(1,820)	778
Balance	(18,685)	(21,040)	(19,220)
Accumulated Other Comprehensive Income (Loss) [Member] | Accounting Standards Update 2018-02 [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Reclassification from adoption of ASU 2018-02	$ (352)
Unrealized Gains (Losses) from Postretirement Plan [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance		831	916
Other comprehensive income (loss) before reclassifications		(831)	120
Reclassification adjustment for gains realized in income			(205)
Total other comprehensive income (loss)		(831)	(85)
Balance			831
Disproportionate Tax Effect from Postretirement Plan [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance		511	511
Other comprehensive income (loss) before reclassifications		(511)
Total other comprehensive income (loss)		$ (511)
Balance			$ 511