Borrowings - Schedule of Remaining Borrowing Capacity with FHLB (Parenthetical) (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Interest Rate 1.54% and 0.74% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Fixed rate on borrowings	1.54%	0.74%
Interest Rate 1.54% and 0.74% [Member] | Maximum [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings	$ 150,000,000
Interest Rate 0.83% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Fixed rate on borrowings	0.83%
Interest Rate 1.25% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Fixed rate on borrowings	1.25%
Interest Rate 1.05% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Fixed rate on borrowings	1.05%
Interest Rate 1.66% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Fixed rate on borrowings	1.66%
Interest Rate 1.17% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Fixed rate on borrowings	1.17%
Interest Rate 1.35% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Fixed rate on borrowings	1.35%
Interest Rate 1.82% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Fixed rate on borrowings	1.82%
Interest Rate 1.27% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Fixed rate on borrowings	1.27%
Interest Rate 1.97% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Fixed rate on borrowings	1.97%
Interest Rate 1.97% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Fixed rate on borrowings	1.97%
Interest Rate 1.47% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Fixed rate on borrowings	1.47%
Interest Rate 2.39% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Fixed rate on borrowings	2.39%
Interest Rate 4.72% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Fixed rate on borrowings	4.72%
Interest Rate 4.31% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Fixed rate on borrowings	4.31%
Interest Rate 3.05% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Fixed rate on borrowings	3.05%
Interest Rate 5.24% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Fixed rate on borrowings	5.24%
Interest Rate 3.35% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Fixed rate on borrowings	3.35%
Interest Rate 4.00% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Fixed rate on borrowings	4.00%
Interest Rate 2.96% [Member]
Borrowings Under Repurchase Agreements [Line Items]
Fixed rate on borrowings	2.96%