Other Comprehensive Income - Amounts Reclassified Out of Each Component of Accumulated Other 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 [Line Items]
Net gain (losses) on securities available for sale	$ (18)	$ 5	$ (20)	$ 5
Income tax expense	1,304	1,159	5,251	3,414
Net of tax	(3,306)	(2,862)	(12,430)	(8,360)
Reclassifications, net of tax	(43)	(46)	(152)	(138)
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, net of tax	12		13
Unrealized Gains (Losses) on Available-for-Sale Securities [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain (losses) on securities available for sale	18		20
Income tax expense	(6)		(7)
Net of tax	12		13
Accumulated Defined Benefit Plans Adjustment, Actuarial Gains/(Losses) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries, wages and benefits	(83)	(70)	(249)	(210)
Defined Benefit Pension Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	28	24	84	72
Reclassifications, net of tax	$ (55)	$ (46)	$ (165)	$ (138)