Other Comprehensive Income (Loss) - Summary of Reclassification Out of Each Component of Accumulated Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income On Derivatives [Line Items]
Net gains on securities available for sale	$ 233		$ 386	$ 11
Tax expense	(2,529)	$ (2,040)	(4,121)	(3,855)
Net income	5,330	$ 4,125	8,650	7,811
Amount Reclassified From Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income On Derivatives [Line Items]
Net income	333		613
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	233		386	11
Tax expense	(81)		(135)	(4)
Net income	152		251	$ 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	278		556
Tax expense	(97)		(194)
Net income	$ 181		$ 362