Reclassifications Out of Accumulated Other Comprehensive Income - Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gains on sales of securities	$ 19	$ 52	$ 64	$ 170
Provision for income taxes	52	(180)	(32)	(628)
Net income	6,486	6,166	18,034	17,050
Salaries and employee benefits	(10,544)	(10,087)	(30,360)	(28,701)
Reclassification Out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	(757)	(1,117)	(2,962)	(3,234)
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	19	52	64	170
Provision for income taxes	(7)	(21)	(25)	(68)
Net income	12	31	39	102
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	(809)	(1,436)	(3,490)	(4,141)
Provision for income taxes	282	501	1,216	1,444
Net income	(527)	(935)	(2,274)	(2,697)
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	(3)	(2)	(8)	(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	(401)	(352)	(1,203)	(1,056)
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	(404)	(354)	(1,211)	(1,064)
Provision for income taxes	162	141	484	425
Net income	$ (242)	$ (213)	$ (727)	$ (639)