Securities - Summary of Amortized Cost and Estimated Fair Values of Securities Held for Investment (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 2,008	$ 2,292
Gross Unrealized Gains	9	13
Gross Unrealized Losses		(2)
Estimated Fair Value	2,017	2,303
Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	158	187
Gross Unrealized Gains	6	10
Estimated Fair Value	164	197
States and Political Subdivisions [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,350	1,605
Gross Unrealized Gains	3	3
Gross Unrealized Losses		(2)
Estimated Fair Value	1,353	1,606
Other Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	500	500
Estimated Fair Value	$ 500	$ 500