Securities - Summary of Amortized Cost and Estimated Fair Values of Debt Securities Held for Investment (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 2,964	$ 1,903
Gross Unrealized Gains	21	5
Gross Unrealized Losses	(1)	(5)
Estimated Fair Value	2,984	1,903
Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	59	93
Gross Unrealized Gains	3	4
Estimated Fair Value	62	97
States and Political Subdivisions [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,405	1,310
Gross Unrealized Gains	18	1
Gross Unrealized Losses	(1)	(5)
Estimated Fair Value	2,422	1,306
Other Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	500	500
Estimated Fair Value	$ 500	$ 500