Comprehensive Income - Components of Other Comprehensive Income and Related Tax Effects (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Equity [Abstract]
Unrealized holding gains on available for sale securities before reclassifications	$ 4,911	$ 7,173	$ 5,698	$ 13,298
Amounts reclassified out of accumulated other comprehensive income	0	0	0	0
Unrealized holding gains on available for sale securities after reclassifications	4,911	7,173	5,698	13,298
Tax effect	(2,065)	(3,016)	(2,395)	(5,591)
Unrealized holding gains on available for sale securities, net of tax	2,846	4,157	3,303	7,707
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)	(20)	(4)	(20)
Amortization of actuarial losses	96	275	192	275
Total amounts reclassified out of accumulated other comprehensive income	95	255	188	255
Change in unfunded status of the supplemental retirement plans after reclassifications	95	255	188	255
Tax effect	(40)	(107)	(79)	(107)
Change in unfunded status of the supplemental retirement plans, net of tax	55	148	109	148
Change in joint beneficiary agreement liability before reclassifications	0	(4)	0	(4)
Amounts reclassified out of accumulated other comprehensive income	0	0	0	0
Change in joint beneficiary agreement liability after reclassifications	0	(4)	0	(4)
Tax effect	0	0	0	0
Change in joint beneficiary agreement liability, net of tax	0	(4)	0	(4)
Total other comprehensive income (loss)	$ 2,901	$ 4,301	$ 3,412	$ 7,851