Comprehensive Income - Components of Other Comprehensive Income (Loss) and Related Tax Effects (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized holding gains (losses) on available for sale securities before reclassifications	$ (5,676)	$ 4,911	$ (20,941)	$ 5,698
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	(5,676)	4,911	(21,304)	5,698
Tax effect	1,678	(2,065)	6,280	(2,395)
Unrealized holding gains (losses) on available for sale securities, net of tax	(3,998)	2,846	(15,024)	3,303
Change in unfunded status of the supplemental retirement plans before reclassifications			668
Amounts reclassified out of accumulated other comprehensive income:
Amortization of prior service cost	(13)	(1)	(27)	(4)
Amortization of actuarial losses	127	96	254	192
Total amounts reclassified out of accumulated other comprehensive income	114	95	(441)	188
Change in unfunded status of the supplemental retirement plans after reclassifications	114	95	227	188
Tax effect	(34)	(40)	(67)	(79)
Change in unfunded status of the supplemental retirement plans, net of tax	80	55	160	109
Change in joint beneficiary agreement liability before reclassifications	0	0	0	0
Amounts reclassified out of accumulated other comprehensive income	0	0	0	0
Change in joint beneficiary agreement liability after reclassifications	0	0	0	0
Tax effect	0	0	0	0
Change in joint beneficiary agreement liability, net of tax	0	0	0	0
Other comprehensive income (loss)	$ (3,918)	$ 2,901	(14,864)	$ 3,412
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)