Securities - Summary of Amortized Cost and Estimated Fair Values of Debt Securities Held for Investment (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,217	$ 1,428
Gross Unrealized Gains	6	5
Estimated Fair Value	1,223	1,433
Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	122	133
Gross Unrealized Gains	6	5
Estimated Fair Value	128	138
States and Political Subdivisions [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	595	795
Estimated Fair Value	595	795
Other Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	500	500
Estimated Fair Value	$ 500	$ 500