Bank Premises and Equipment - Components of Bank Premises and Equipment (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Bank premises and equipment, gross	$ 28,983	$ 27,440
Accumulated depreciation and amortization	(17,028)	(14,671)
Total	11,955	12,769
Leasehold improvements
Property, Plant and Equipment [Line Items]
Bank premises and equipment, gross	$ 14,028	13,690
Leasehold improvements | Minimum
Property, Plant and Equipment [Line Items]
Expected useful life (years)	3 years
Leasehold improvements | Maximum
Property, Plant and Equipment [Line Items]
Expected useful life (years)	25 years
Furniture, fixtures and equipment
Property, Plant and Equipment [Line Items]
Bank premises and equipment, gross	$ 6,447	6,138
Furniture, fixtures and equipment | Minimum
Property, Plant and Equipment [Line Items]
Expected useful life (years)	5 years
Furniture, fixtures and equipment | Maximum
Property, Plant and Equipment [Line Items]
Expected useful life (years)	10 years
IT equipment
Property, Plant and Equipment [Line Items]
Bank premises and equipment, gross	$ 8,002	7,106
IT equipment | Minimum
Property, Plant and Equipment [Line Items]
Expected useful life (years)	3 years
IT equipment | Maximum
Property, Plant and Equipment [Line Items]
Expected useful life (years)	5 years
Automobiles
Property, Plant and Equipment [Line Items]
Bank premises and equipment, gross	$ 506	$ 506
Automobiles | Minimum
Property, Plant and Equipment [Line Items]
Expected useful life (years)	3 years
Automobiles | Maximum
Property, Plant and Equipment [Line Items]
Expected useful life (years)	5 years