Securities - Securities with Unrealized Losses Not Recognized in Income (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
12 Months or less, Fair Value	$ 40,292	$ 65,891
12 Months or less, Unrealized Loss	(207)	(1,185)
More than 12 months, Fair Value	24,285	4,261
More than 12 months, Unrealized Loss	(461)	(63)
Total Fair Value	64,577	70,152
Total Unrealized Loss	(668)	(1,248)
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	13,574	13,271
12 Months or less, Unrealized Loss	(48)	(61)
More than 12 months, Fair Value	3,761	893
More than 12 months, Unrealized Loss	(39)	(16)
Total Fair Value	17,335	14,164
Total Unrealized Loss	(87)	(77)
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
12 Months or less, Fair Value	4,960	17,167
12 Months or less, Unrealized Loss	(51)	(558)
More than 12 months, Fair Value	5,462	519
More than 12 months, Unrealized Loss	(174)	(16)
Total Fair Value	10,422	17,686
Total Unrealized Loss	(225)	(574)
Mortgage-backed Securities in Government Sponsored Entities [Member]
Schedule of Available-for-sale Securities [Line Items]
12 Months or less, Fair Value	21,758	35,453
12 Months or less, Unrealized Loss	(108)	(566)
More than 12 months, Fair Value	15,062	2,849
More than 12 months, Unrealized Loss	(248)	(31)
Total Fair Value	36,820	38,302
Total Unrealized Loss	$ (356)	$ (597)