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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income On Derivatives [Line Items]
Net gains on securities available for sale									$ 142	$ 444	$ 2,577
Tax expense (benefit)	$ (1,965)	$ (2,073)	$ (2,040)	$ (1,815)	$ 685	$ 369	$ 38,837	$ (156)	(7,893)	39,735	(200)
Net income	$ 4,328	$ 4,143	$ 4,125	$ 3,686	$ 2,808	$ 2,900	$ 42,404	$ 2,094	16,282	50,206	10,027
Increase to net income									(297)	(432)	(2,766)
Unrealized Gains (Losses) from Postretirement Plan [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income On Derivatives [Line Items]
Increase to net income									(205)	(143)	(189)
Amount Reclassified From Accumulated Other Comprehensive Income to Net Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income On Derivatives [Line Items]
Increase to net income									(297)	(432)	(2,766)
Amount Reclassified From Accumulated Other Comprehensive Income to Net Income [Member] | Realized Net Gains on Sale of Available for Sale Securities [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income On Derivatives [Line Items]
Net gains on securities available for sale									(142)	(444)	(2,577)
Tax expense (benefit)									50	155
Net income									(92)	(289)	(2,577)
Amount Reclassified From Accumulated Other Comprehensive Income to Net Income [Member] | Unrealized Gains (Losses) from Postretirement Plan [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income On Derivatives [Line Items]
Reduction of salaries & employee benefits expense									(315)	(220)	(189)
Tax expense (benefit)									110	77
Net income									$ (205)	$ (143)	$ (189)