Off Balance Sheet Activities - Schedule of Off-Balance Sheet Risks (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Commitments to Make Loans [Member] | Fixed Rate [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input [Line Items]
Off-balance sheet risks, amount, liability	$ 64,799	$ 57,283
Commitments to Make Loans [Member] | Variable Rate [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input [Line Items]
Off-balance sheet risks, amount, liability	210,987	202,883
Unused Lines of Credit [Member] | Fixed Rate [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input [Line Items]
Off-balance sheet risks, amount, liability	0	0
Unused Lines of Credit [Member] | Variable Rate [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input [Line Items]
Off-balance sheet risks, amount, liability	118,348	105,779
Standby Letters of Credit [Member] | Fixed Rate [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input [Line Items]
Off-balance sheet risks, amount, liability	0	0
Standby Letters of Credit [Member] | Variable Rate [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input [Line Items]
Off-balance sheet risks, amount, liability	$ 14,985	$ 4,618