Reclassifications Out of Accumulated Other Comprehensive Income - Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gains on sales of securities	$ 53	$ 105	$ 61	$ 302
Provision for income taxes	(435)	(444)	(584)	(1,259)
Net income	10,084	9,581	28,967	26,288
Salaries and employee benefits	(10,670)	(10,570)	(32,621)	(32,331)
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gains on sales of securities	54	105	61	302
Provision for income taxes	(15)	(29)	(17)	(85)
Net income	39	76	44	217
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Accretion of Unrealized Losses Transferred [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gains on sales of securities	(209)	(323)	(779)	(1,186)
Provision for income taxes	54	85	205	314
Net income	(155)	(238)	(574)	(872)
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Prior-Service Costs [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits	(29)	(4)	(86)	(12)
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Actuarial Gains (Losses) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits	(337)	(402)	(1,013)	(1,207)
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Amortization of Defined Benefit Pension Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before taxes	(366)	(406)	(1,099)	(1,219)
Provision for income taxes	103	114	309	342
Net income	$ (263)	$ (292)	$ (790)	$ (877)