Securities - Summary of Amortized Cost and Estimated Fair Values of Securities Held for Investment (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 2,079	$ 2,292
Gross Unrealized Gains	8	13
Gross Unrealized Losses	(1)	(2)
Estimated Fair Value	2,086	2,303
Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	174	187
Gross Unrealized Gains	7	10
Estimated Fair Value	181	197
States and Political Subdivisions [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,405	1,605
Gross Unrealized Gains	1	3
Gross Unrealized Losses	(1)	(2)
Estimated Fair Value	1,405	1,606
Other Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	500	500
Estimated Fair Value	$ 500	$ 500