Comprehensive Income - Components of Other Comprehensive Income and Related Tax Effects (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Unrealized holding losses on available for sale securities before reclassifications	$ 6,422	$ (11,015)	$ (1,895)
Amounts reclassified out of accumulated other comprehensive income	(961)
Unrealized holding losses on available for sale securities after reclassifications	5,461	(11,015)	(1,895)
Tax effect	(2,296)	4,631	797
Unrealized holding losses on available for sale securities, net of tax	3,165	(6,384)	(1,098)
Change in unfunded status of the supplemental retirement plans before reclassifications	(1,016)	511	1,384
Amounts reclassified out of accumulated other comprehensive income:
Amortization of prior service cost	(12)	(40)	(57)
Amortization of actuarial losses	390	550	823
Total amounts reclassified out of accumulated other comprehensive income	378	510	766
Change in unfunded status of the supplemental retirement plans after reclassifications	(638)	1,021	2,150
Tax effect	268	(429)	(904)
Change in unfunded status of the supplemental retirement plans, net of tax	(370)	592	1,246
Change in joint beneficiary agreement liability before reclassifications	(110)	(343)	277
Amounts reclassified out of accumulated other comprehensive income	0	0	0
Change in joint beneficiary agreement liability after reclassifications	(110)	(343)	277
Tax effect	0	0	0
Change in joint beneficiary agreement liability, net of tax	(110)	(343)	277
Total other comprehensive income (loss)	$ 2,685	$ (6,135)	$ 425