Securities - Securities with Unrealized Losses Not Recognized in Income (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
12 Months or less, Fair Value	$ 99,664	$ 54,040
12 Months or less, Unrealized Loss	(1,634)	(336)
More than 12 months, Fair Value	38,540	36,125
More than 12 months, Unrealized Loss	(1,159)	(718)
Total Fair Value	138,204	90,165
Total Unrealized Loss	(2,793)	(1,054)
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
12 Months or less, Fair Value	15,172	20,449
12 Months or less, Unrealized Loss	(159)	(100)
More than 12 months, Fair Value	11,366	6,617
More than 12 months, Unrealized Loss	(135)	(92)
Total Fair Value	26,538	27,066
Total Unrealized Loss	(294)	(192)
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
12 Months or less, Fair Value	27,449	4,057
12 Months or less, Unrealized Loss	(448)	(41)
More than 12 months, Fair Value	7,453	7,309
More than 12 months, Unrealized Loss	(253)	(131)
Total Fair Value	34,902	11,366
Total Unrealized Loss	(701)	(172)
Mortgage-backed Securities in Government Sponsored Entities [Member]
Schedule of Available-for-sale Securities [Line Items]
12 Months or less, Fair Value	57,043	29,534
12 Months or less, Unrealized Loss	(1,027)	(195)
More than 12 months, Fair Value	19,721	22,199
More than 12 months, Unrealized Loss	(771)	(495)
Total Fair Value	76,764	51,733
Total Unrealized Loss	$ (1,798)	$ (690)