Quantitative Information About Unobservable Inputs Used in Recurring and Nonrecurring Level 3 Fair Value Measurements (Detail) - Market comparable properties [Member] - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Collateral-dependent impaired loans | Marketability discount [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 334	$ 336
Foreclosed assets held for sale | Selling costs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 151	$ 34
Foreclosed assets held for sale | Maximum | Selling costs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range	35.00%	35.00%
Foreclosed assets held for sale | Minimum | Selling costs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range	10.00%	10.00%