Other Comprehensive Income - Amounts Reclassified Out of Each Component of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain (loss) on sale of securities	$ 92	$ 3	$ 92	$ 17
Other operating expenses	(2,197)	(2,323)	(7,115)	(6,843)
Income tax expense	(1,133)	(1,258)	(3,134)	(4,688)
Net Income	7,682	7,708	22,019	26,038
Reclassification out of Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income	16	(114)	(98)	(335)
Unrealized Gains (Losses) on Available-for-Sale Securities [Member] | Reclassification out of Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain (loss) on sale of securities	92	3	92	17
Income tax expense	(19)	(1)	(19)	(4)
Net Income	73	2	73	13
Accumulated Defined Benefit Plans Adjustment, Actuarial gains/(Losses) [Member] | Reclassification out of Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other operating expenses	(72)	(147)	(217)	(441)
Income tax expense	15	31	46	93
Net Income	$ (57)	$ (116)	$ (171)	$ (348)