Supplemental Comprehensive Income Disclosures - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplemental Comprehensive Income Disclosures [Abstract]
Unrealized holding (losses) gains on securities arising during the period, Pretax Amount	$ (6,304)	$ 2,798	$ 25,153
Less: reclassification adjustment for (gains) losses on securities included in net income, Pretax Amount	(617)	153	(550)
Total Unrealized (Losses) Gains on Securities, before Tax	(6,921)	2,951	24,603
Unrealized holding gains on derivatives arising during the period, Pretax Amount	(479)	450	472
Reclassification adjustment for gains on derivatives included in net income, Pretax Amount	70	49	10
Total unrealized gains on derivatives, Pretax Amount	(549)	401	462
Unrealized holding gains on derivatives arising during the period, Tax (Expense) Benefit	168	(158)	(165)
Reclassification adjustment for gains on derivatives included in net income, Tax (Expense) Benefit	24	17	3
Total unrealized gains on derivatives, Tax (Expense) Benefit	192	(141)	(162)
Unrealized holding gains on derivatives arising during the period, Net of Tax Amount	(311)	292	307
Reclassification adjustment for gains on derivatives included in net income, Tax (Expense) Benefit	(46)	(32)	(7)
Total unrealized gains on derivatives, Tax (Expense) Benefit	(357)	260	300
Transition obligation	0	0	0
Net (loss) gain, Pretax Amount	331	(102)	(313)
Total Unrealized Gains (Losses) for Postretirement Obligations, Before Tax	331	(102)	(313)
Total other comprehensive (loss) income, Pretax Amount	(7,139)	3,250	24,752
Unrealized holding (losses) gains on securities arising during the period, Tax (Expense) Benefit	2,206	(978)	(8,803)
Less: reclassification adjustment for (gains) losses on securities included in net income, Tax (Expense) Benefit	216	(54)	193
Total Unrealized (Losses) Gains on Securities, Tax (Expense) Benefit	2,422	(1,032)	(8,610)
Transition obligation, Tax (Expense) Benefit	0	0	0
Net (loss) gain, Tax (Expense) Benefit	(116)	36	109
Total Unrealized Gains (Losses) for Postretirement Obligations, Tax (Expense) Benefit	(116)	36	109
Total other comprehensive (loss) income, Tax (Expense) Benefit	2,498	(1,137)	(8,663)
Unrealized holding (losses) gains on securities arising during the period, Net of Tax Amount	(4,098)	1,820	16,350
Less: reclassification adjustment for (gains) losses on securities included in net income, Net of Tax Amount	(401)	99	(357)
Total Unrealized (Losses) Gains on Securities, Net of Tax	(4,499)	1,919	15,993
Transition obligation, Net of Tax Amount	0	0	0
Net gain	215	(66)	(204)
Total Unrealized Gains (Losses) for Postretirement Obligations, Net of Tax	215	(66)	(204)
Total other comprehensive (loss) Income	$ (4,641)	$ 2,113	$ 16,089