Comprehensive Income - Components of Other Comprehensive Income and Related Tax Effects (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized holding gains (losses) on available for sale securities before reclassifications	$ (15,265)	$ 787
Amounts reclassified out of accumulated other comprehensive income:
Total amounts reclassified out of accumulated other comprehensive income	(363)
Unrealized holding gains (losses) on available for sale securities after reclassifications	(15,628)	787
Tax effect	4,602	(330)
Unrealized holding gains (losses) on available for sale securities, net of tax	(11,026)	457
Change in unfunded status of the supplemental retirement plans before reclassifications	667
Amounts reclassified out of accumulated other comprehensive income:
Amortization of prior service cost	(13)	(3)
Amortization of actuarial losses	127	96
Total amounts reclassified out of accumulated other comprehensive income	(554)	93
Change in unfunded status of the supplemental retirement plans after reclassifications	113	93
Tax effect	(33)	(39)
Change in unfunded status of the supplemental retirement plans, net of tax	80	54
Change in joint beneficiary agreement liability before reclassifications	0	0
Amounts reclassified out of accumulated other comprehensive income	0	0
Change in joint beneficiary agreement liability after reclassifications	0	0
Tax effect	0	0
Change in joint beneficiary agreement liability, net of tax	0	0
Other comprehensive income (loss)	(10,946)	$ 511
ASU 2016-01 [Member]
Amounts reclassified out of accumulated other comprehensive income:
Total amounts reclassified out of accumulated other comprehensive income	62
ASU 2018-02 [Member]
Amounts reclassified out of accumulated other comprehensive income:
Total amounts reclassified out of accumulated other comprehensive income	(425)
Amounts reclassified out of accumulated other comprehensive income:
Total amounts reclassified out of accumulated other comprehensive income	$ (668)