Bank Premises and Equipment - Summary of Bank Premises and Equipment (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Land	$ 31,190	$ 29,508
Buildings	135,335	119,728
Furniture and equipment	58,969	58,672
Leasehold improvements	3,557	4,118
Gross	229,051	212,026
Less- accumulated depreciation and amortization	(95,630)	(88,000)
Total Bank Premises and Equipment	$ 133,421	$ 124,026
Minimum [Member] | Buildings [Member]
Property, Plant and Equipment [Line Items]
Useful Life in years	20 years
Minimum [Member] | Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Useful Life in years	3 years
Minimum [Member] | Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Useful Life in years	5 years
Maximum [Member] | Buildings [Member]
Property, Plant and Equipment [Line Items]
Useful Life in years		40 years
Maximum [Member] | Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Useful Life in years		10 years
Maximum [Member] | Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Useful Life in years		15 years