Supplemental Comprehensive Income Disclosures - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Supplemental Comprehensive Income Disclosures [Abstract]
Unrealized holding (losses) gains on securities arising during the period, Pretax Amount	$ 20,625	$ 2,783	$ 7,023
Less: reclassification adjustment for (gains) losses on securities included in net income, Pretax Amount	(22)	(8,102)	(5,040)
Total Unrealized (Losses) Gains on Securities, before Tax	20,603	(5,319)	1,983
Unrealized holding gains on derivatives arising during the period, Pretax Amount	935	326	(901)
Reclassification adjustment for gains on derivatives included in net income, Pretax Amount	0	(10)	(119)
Total unrealized gains on derivatives, Pretax Amount	935	336	(782)
Unrealized holding gains on derivatives arising during the period, Tax (Expense) Benefit	(196)	(68)	315
Reclassification adjustment for gains on derivatives included in net income, Tax (Expense) Benefit	0	(3)	(42)
Total unrealized gains on derivatives, Tax (Expense) Benefit	(196)	(71)	273
Unrealized holding gains on derivatives arising during the period, Net of Tax Amount	739	258	(586)
Reclassification adjustment for gains on derivatives included in net income, Tax (Expense) Benefit	0	7	77
Total unrealized gains on derivatives, Tax (Expense) Benefit	739	265	(509)
Net (loss) gain, Pretax Amount	(121)	144	94
Total Unrealized Gains (Losses) for Postretirement Obligations, Before Tax	(121)	144	94
Total other comprehensive (loss) income, Pretax Amount	21,417	(4,839)	1,295
Unrealized holding (losses) gains on securities arising during the period, Tax (Expense) Benefit	(4,331)	(585)	(2,458)
Less: reclassification adjustment for (gains) losses on securities included in net income, Tax (Expense) Benefit	5	1,701	1,764
Total Unrealized (Losses) Gains on Securities, Tax (Expense) Benefit	(4,326)	1,116	(694)
Net (loss) gain, Tax (Expense) Benefit	25	(30)	(20)
Total Unrealized Gains (Losses) for Postretirement Obligations, Tax (Expense) Benefit	25	(30)	(20)
Total other comprehensive (loss) income, Tax (Expense) Benefit	(4,497)	1,015	(441)
Unrealized holding (losses) gains on securities arising during the period, Net of Tax Amount	16,294	2,198	4,565
Less: reclassification adjustment for (gains) losses on securities included in net income, Net of Tax Amount	(17)	(6,401)	(3,276)
Total Unrealized (Losses) Gains on Securities, Net of Tax	16,277	(4,203)	1,289
Net gain	(96)	114	74
Total Unrealized Gains (Losses) for Postretirement Obligations, Net of Tax	(96)	114	74
Total other comprehensive (loss) Income	$ 16,920	$ (3,824)	$ 854