Reclassifications Out of Accumulated Other Comprehensive Income - Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gains on sales of securities	$ 7		$ 7	$ 197
Provision for income taxes	(267)	$ (314)	(149)	(815)
Net income	9,466	8,998	18,884	16,707
Salaries and employee benefits	(10,916)	(10,536)	(21,951)	(21,761)
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	7		7	197
Provision for income taxes	(2)		(2)	(55)
Net income	5		5	142
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	(276)	(343)	(570)	(863)
Provision for income taxes	73	90	151	229
Net income	(203)	(253)	(419)	(634)
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)	(58)	(8)
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	(338)	(402)	(675)	(805)
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	(367)	(406)	(733)	(813)
Provision for income taxes	103	114	206	228
Net income	$ (264)	$ (292)	$ (527)	$ (585)