Fair Value Measurements - Assets Measured at Fair Value (Detail) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2019 USD ($)		Dec. 31, 2018 USD ($)
Securities AFS [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Fair Value		$ 63,332	[1]	$ 88,728
Valuation Technique	[1]	Discounted cash flow		Discounted cash flow
Unobservable Input	[1]	Discount rate		Discount rate
Impaired Loans [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Fair Value		$ 195		$ 251
Valuation Technique		Appraisal of collateral		Appraisal of collateral
Unobservable Input		Appraisal adjustments		Appraisal adjustments
Other Real Estate Owned [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Fair Value		$ 2,225		$ 2,225
Valuation Technique		Appraisal of collateral		Appraisal of collateral
Unobservable Input		Appraisal adjustments		Appraisal adjustments
Unobservable Input Value or Range		30.00%		30.00%
Minimum [Member] | Securities AFS [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities AFS, Unobservable Input Value or Range		2.1		2.1
Minimum [Member] | Impaired Loans [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Impaired Loans, Unobservable Input Value or Range		0		0
Maximum [Member] | Securities AFS [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities AFS, Unobservable Input Value or Range		3.8		4.1
Maximum [Member] | Impaired Loans [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Impaired Loans, Unobservable Input Value or Range		30		30

[1] Municipal securities generally have maturities of one year or less and, therefore, the amortized cost equates to the fair value.