Other Comprehensive Income - Amounts Reclassified Out of Each Component of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain on securities available for sale	$ (4)
Other operating expenses	(2,477)	$ (1,758)
Income tax expense	(1,885)	(1,194)
Net Income	9,669	6,989
Reclassification out of Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income	(119)	(113)
Unrealized gains on available-for-sale securities [Member] | Reclassification out of Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain on securities available for sale	(4)
Income tax expense	1
Net Income	(3)
Accumulated Defined Benefit Plans Adjustment, Actuarial gains/(Losses) [Member] | Reclassification out of Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other operating expenses	(147)	(143)
Income tax expense	31	30
Net Income	$ (116)	$ (113)