Borrowings - Schedule of Remaining Borrowing Capacity with FHLB (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 257,359	$ 227,004
Interest Rate 1.54% and 0.74% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	34,416	103,969
Interest Rate 0.83% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 0	15,016
Maturity date	Sep. 06, 2017
Interest Rate 1.25% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 0	15,093
Maturity date	Dec. 15, 2017
Interest Rate 1.05% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 7,550	17,523
Maturity date	Sep. 06, 2018
Interest Rate 1.66% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 30,000	0
Maturity date	Jun. 28, 2019
Interest Rate 1.17% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 11,752	18,360
Maturity date	Sep. 06, 2019
Interest Rate 1.35% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 8,883	13,865
Maturity date	Sep. 16, 2019
Interest Rate 1.82% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 30,000	0
Maturity date	Jun. 29, 2020
Interest Rate 1.27% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 13,859	18,780
Maturity date	Sep. 08, 2020
Interest Rate 1.97% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 30,000	0
Maturity date	Jun. 28, 2021
Interest Rate 1.97% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 50,000	0
Maturity date	Jun. 29, 2022
Interest Rate 1.47% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 15,976	19,201
Maturity date	Sep. 06, 2022
Interest Rate 2.39% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 20,000	0
Maturity date	Dec. 05, 2022
Interest Rate 4.72% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 700	700
Maturity date	Jul. 03, 2023
Interest Rate 4.31% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 1,797	1,925
Maturity date	Sep. 05, 2023
Interest Rate 3.05% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 814	852
Maturity date	Mar. 11, 2024
Interest Rate 5.24% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 435	462
Maturity date	Jun. 11, 2024
Interest Rate 3.35% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 217	240
Maturity date	May 04, 2026
Interest Rate 4.00% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 231	252
Maturity date	Oct. 14, 2026
Interest Rate 2.96% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 729	$ 766
Maturity date	Feb. 11, 2033