Securities - Summary of Amortized Cost and Estimated Fair Values of Debt Securities Held for Investment (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 2,956	$ 2,964
Gross Unrealized Gains	6	21
Gross Unrealized Losses	(9)	(1)
Estimated Fair Value	2,953	2,984
Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	51	59
Gross Unrealized Gains	3	3
Estimated Fair Value	54	62
States and Political Subdivisions [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,405	2,405
Gross Unrealized Gains	3	18
Gross Unrealized Losses	(9)	(1)
Estimated Fair Value	2,399	2,422
Other Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	500	500
Estimated Fair Value	$ 500	$ 500