Other Comprehensive Income - Amounts Reclassified Out of Each Component of Accumulated Other 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 [Line Items]
Income tax expense	$ 2,084	$ 1,113	$ 3,947	$ 2,255
Net of tax	(4,790)	(2,731)	(9,124)	(5,497)
Reclassifications, net of tax	(59)	(46)	(111)	(92)
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, net of tax	(4)		(1)
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 on securities available for sale	(6)		(1)
Income tax expense	2
Net of tax	(4)		(1)
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)	(166)	(140)
Defined Benefit Pension Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	28	24	56	48
Reclassifications, net of tax	$ (55)	$ (46)	$ (110)	$ (92)