Reclassifications Out of Accumulated Other Comprehensive Income - Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gains on sales of securities									$ 0	$ 61	$ 302
Provision for income taxes	$ (1,203)	$ (1,546)	$ (1,061)	$ (597)	$ (526)	$ (435)	$ (267)	$ 118	(4,407)	(1,110)	(1,568)
Net income	$ 11,600	$ 10,887	$ 10,056	$ 9,666	$ 10,732	$ 10,084	$ 9,466	$ 9,418	42,209	39,699	36,213
Salaries and employee benefits									(45,303)	(44,014)	$ (42,710)
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										61
Provision for income taxes										(17)
Net income										44
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									(798)	(1,022)
Provision for income taxes									207	269
Net income									(591)	(753)
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									(114)	(114)
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									(1,889)	(1,351)
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									(2,003)	(1,465)
Provision for income taxes									562	412
Net income									$ (1,441)	$ (1,053)