Shareholders' Equity - Summary of Reclassification Out of Each Component of Accumulated Comprehensive Income (Loss) to Net Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income On Derivatives [Line Items]
Net gains on securities available for sale									$ 566	$ 604	$ 142
Reduction of salaries & employee benefits expense									0	1,278	315
Tax (expense) benefit	$ (4,294)	$ (3,067)	$ (3,377)	$ (557)	$ (1,734)	$ (2,288)	$ (2,529)	$ (1,592)	(11,295)	(8,143)	(7,893)
Net income	$ 4,502	$ 7,556	$ 8,189	$ 1,538	$ 5,025	$ 5,153	$ 5,330	$ 3,320	21,785	18,828	16,282
Amount Reclassified From Accumulated Other Comprehensive Income to Net Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income On Derivatives [Line Items]
Net income									368	1,735	297
Amount Reclassified From Accumulated Other Comprehensive Income to Net 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									566	604	142
Tax (expense) benefit									$ (198)	(211)	(50)
Net income										393	92
Amount Reclassified From Accumulated Other Comprehensive Income to Net Income [Member] | Amortization of Postretirement Benefits Prior Service Costs [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income On Derivatives [Line Items]
Reduction of salaries & employee benefits expense										1,278	315
Tax (expense) benefit										(447)	(110)
Disproportionate tax benefit effect from plan settlement										511
Net income										$ 1,342	$ 205