Supplemental Comprehensive Income Disclosures - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized holding gains on securities arising during the period	$ 19,981	$ 20,625	$ 2,783
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax	4,196	4,331	585
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax	15,785	16,294	2,198
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	70	22	8,102
Less: reclassification adjustment for (gains) losses on securities included in net income, Tax (Expense) Benefit	15	5	1,701
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	55	17	6,401
Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, before Tax	19,911	20,603	(5,319)
Other Comprehensive Income (Loss), Securities, Available-for-sale, Tax	4,181	4,326	(1,116)
Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax	15,730	16,277	(4,203)
Unrealized holding (losses) gains on derivatives arising during the period	(4,467)	935	326
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax	(938)	196	68
Unrealized holding gains on derivatives arising during the period, Net of Tax Amount	(3,529)	739	258
Reclassification adjustment for gains on derivatives included in net income, Pretax Amount	0	0	(10)
Reclassification adjustment for gains on derivatives included in net income, Tax (Expense) Benefit	0	0	(3)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0	0	(7)
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax	(4,467)	935	336
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Tax	(938)	196	71
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	(3,529)	739	265
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax	(537)	0	0
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, Tax	113	0	0
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, after Tax	424	0	0
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax	155	121	(144)
Net (loss) gain, Tax (Expense) Benefit	32	25	(30)
Net gain	(123)	(96)	114
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, before Tax	692	121	(144)
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax	145	25	(30)
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	547	96	(114)
Total other comprehensive (loss) income, Pretax Amount	14,752	21,417	(4,839)
Other Comprehensive Income (Loss), Tax	3,098	4,497	(1,015)
Total other comprehensive (loss) Income	11,654	16,920	(3,824)
Post-Retirement Obligation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, after Tax	(424)
Net gain	(123)
Total other comprehensive (loss) Income	$ (547)	$ (96)	$ 114