Other Comprehensive Income (Loss) - Summary of Reclassification Out of Each Component of Accumulated Comprehensive Income (Loss) (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 On Derivatives [Line Items]
Net gains on securities available for sale	$ 218		$ 604	$ 11
Tax expense	(2,288)	$ (2,073)	(6,409)	(5,928)
Net income	5,153	$ 4,143	13,803	11,954
Amount Reclassified From Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income On Derivatives [Line Items]
Net income	431		1,044
Amount Reclassified From Accumulated Other Comprehensive Income [Member] | Unrealized Gains (Losses) on Securities Available for Sale [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income On Derivatives [Line Items]
Net gains on securities available for sale	218		604	11
Tax expense	(76)		(211)	(4)
Net income	142		393	$ 7
Amount Reclassified From Accumulated Other Comprehensive Income [Member] | Amortization of Postretirement Benefits Prior Service Costs [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income On Derivatives [Line Items]
Reduction in salaries and employee benefits	445		1,001
Tax expense	(156)		(350)
Net income	$ 289		$ 651