Securities - Summary of Amortized Cost and Estimated Fair Values of Securities Held for Investment (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,440	$ 2,292
Gross Unrealized Gains	7	13
Gross Unrealized Losses		(2)
Estimated Fair Value	1,447	2,303
Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	145	187
Gross Unrealized Gains	6	10
Estimated Fair Value	151	197
States and Political Subdivisions [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	795	1,605
Gross Unrealized Gains	1	3
Gross Unrealized Losses		(2)
Estimated Fair Value	796	1,606
Other Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	500	500
Estimated Fair Value	$ 500	$ 500