Comprehensive Income - Components of Other Comprehensive Income and Related Tax Effects (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Equity [Abstract]
Unrealized holding gains (losses) on available for sale securities before reclassifications	$ 674	$ (2,058)	$ 6,372	$ 11,240
Amounts reclassified out of accumulated other comprehensive income	(961)		(961)
Unrealized holding (losses) gains on available for sale securities after reclassifications	(287)	(2,058)	5,411	11,240
Tax effect	121	865	(2,274)	(4,726)
Unrealized holding (losses) gains on available for sale securities, net of tax	(166)	(1,193)	3,137	6,514
Change in unfunded status of the supplemental retirement plans before reclassifications	0	0	0	0
Amounts reclassified out of accumulated other comprehensive income:
Amortization of prior service cost	(1)	(10)	(5)	(30)
Amortization of actuarial losses	96	138	288	413
Total amounts reclassified out of accumulated other comprehensive income	95	128	283	383
Change in unfunded status of the supplemental retirement plans after reclassifications	95	128	283	383
Tax effect	(40)	(54)	(119)	(161)
Change in unfunded status of the supplemental retirement plans, net of tax	55	74	164	222
Change in joint beneficiary agreement liability before reclassifications		(1)		(5)
Amounts reclassified out of accumulated other comprehensive income	0	0	0	0
Change in joint beneficiary agreement liability after reclassifications		(1)		(5)
Tax effect	0	0	0	0
Change in joint beneficiary agreement liability, net of tax		(1)		(5)
Total other comprehensive (loss) income	$ (111)	$ (1,120)	$ 3,301	$ 6,731