Securities Available for Sale - Summary of Gross Unrealized Losses on Securities Available-for-sale and Fair Values, Aggregated by Investment category and Length of Time Individual Securities Have Been in a Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 15,152	$ 20,199
Unrealized Loss	(377)	(149)
Fair Value	45,435	42,978
Unrealized Loss	(2,334)	(1,256)
Fair Value	60,587	63,177
Unrealized Loss	(2,711)	(1,405)
US States and Political Subdivisions Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value	2,112	1,435
Unrealized Loss	(16)	(18)
Fair Value	6,289	5,866
Unrealized Loss	(282)	(140)
Fair Value	8,401	7,301
Unrealized Loss	(298)	(158)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value	11,801	18,507
Unrealized Loss	(358)	(131)
Fair Value	39,145	36,176
Unrealized Loss	(2,052)	(1,104)
Fair Value	50,946	54,683
Unrealized Loss	(2,410)	(1,235)
Collateralized Mortgage Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value	160	8
Fair Value	1
Fair Value	161	8
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value	833
Fair Value		936
Unrealized Loss		(12)
Fair Value	833	936
Unrealized Loss		(12)
Certificates of Deposit [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value	246	249
Unrealized Loss	(3)
Fair Value	246	$ 249
Unrealized Loss	$ (3)