Fair Value - Quantitative Information about Significant Unobservable Inputs Used in Fair Value Measurements (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Commercial Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 29,099	$ 26,319
Valuation Techniques	Fair value of collateral	Fair value of collateral
Commercial Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 8,555	$ 2,217
Valuation Techniques	Fair value of collateral	Fair value of collateral
Minimum [Member] | Measurement Input, Discount Rate [Member] | Commercial Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount for type of property, age of appraisal and current status	0.00	0.10
Minimum [Member] | Measurement Input, Discount Rate [Member] | Commercial Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount for type of property, age of appraisal and current status	0.25	0.00
Maximum [Member] | Measurement Input, Discount Rate [Member] | Commercial Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount for type of property, age of appraisal and current status	0.50	0.78
Maximum [Member] | Measurement Input, Discount Rate [Member] | Commercial Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount for type of property, age of appraisal and current status	0.90	0.98
Weighted Average [Member] | Measurement Input, Discount Rate [Member] | Commercial Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount for type of property, age of appraisal and current status	0.30	0.32
Weighted Average [Member] | Measurement Input, Discount Rate [Member] | Commercial Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount for type of property, age of appraisal and current status	0.43	0.49
Pooled Trust Preferred Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 9,072	$ 8,448
Valuation Techniques	Discounted cash flow	Discounted cash flow
Expected asset recoveries	0.00%
Pooled Trust Preferred Securities [Member] | Constant Prepayment Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Constant prepayment rate	0.00%	0.00%
Pooled Trust Preferred Securities [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Additional asset defaults	3.40%	4.20%
Expected asset recoveries		0.00%
Pooled Trust Preferred Securities [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Additional asset defaults	4.40%	9.60%
Expected asset recoveries		4.10%
Pooled Trust Preferred Securities [Member] | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Additional asset defaults	4.10%	7.50%
Expected asset recoveries		0.60%
States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 3,979
Valuation Techniques	Discounted cash flow
Commercial Real Estate Foreclosed Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 543	$ 1,726
Valuation Techniques	Fair value of collateral	Fair value of collateral
Commercial Real Estate Foreclosed Assets [Member] | Minimum [Member] | Measurement Input, Discount Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount for type of property, age of appraisal and current status		0.07
Commercial Real Estate Foreclosed Assets [Member] | Maximum [Member] | Measurement Input, Discount Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount for type of property, age of appraisal and current status		0.25
Commercial Real Estate Foreclosed Assets [Member] | Weighted Average [Member] | Measurement Input, Discount Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount for type of property, age of appraisal and current status	0.12	0.18
Residential Foreclosed Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 151	$ 55
Valuation Techniques	Fair value of collateral	Fair value of collateral
Residential Foreclosed Assets [Member] | Weighted Average [Member] | Measurement Input, Discount Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount for type of property, age of appraisal and current status	0.06	0.39