Fair Value of Assets and Liabilities - Quantitative Information about Level 3 Fair Value Measurements for Assets Measured at Fair Value on Recurring and Non-recurring Basis (Detail) - Significant Unobservable Inputs (Level 3) [Member] $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value, Measurements, Nonrecurring [Member] | Foreclosed Assets [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 3,533	$ 3,369
Unobservable Input	Appraisal comparability adjustments	Appraisal comparability adjustments
Fair Value, Measurements, Nonrecurring [Member] | Impaired Loans [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 386	$ 760
Unobservable Input	Appraisal comparability adjustments	Appraisal comparability adjustments
Fair Value, Measurements, Nonrecurring [Member] | Minimum [Member] | Foreclosed Assets [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Quantitative Range of Unobservable Inputs, Percentage	10	30
Fair Value, Measurements, Nonrecurring [Member] | Minimum [Member] | Impaired Loans [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Quantitative Range of Unobservable Inputs, Percentage	10	10
Fair Value, Measurements, Nonrecurring [Member] | Maximum [Member] | Foreclosed Assets [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Quantitative Range of Unobservable Inputs, Percentage	25	55
Fair Value, Measurements, Nonrecurring [Member] | Maximum [Member] | Impaired Loans [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Quantitative Range of Unobservable Inputs, Percentage	25	15
Fair Value, Measurements, Recurring [Member] | Trust Preferred Securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value		$ 1,685
Unobservable Input		Compare to quotes for sale when available
Quantitative Range of Unobservable Inputs, Description		One month libor
Fair Value, Measurements, Recurring [Member] | Minimum [Member] | Trust Preferred Securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Quantitative Range of Unobservable Inputs, Percentage		4
Fair Value, Measurements, Recurring [Member] | Maximum [Member] | Trust Preferred Securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Quantitative Range of Unobservable Inputs, Percentage		6