Income Taxes - Income Tax Provision Differences from Expected Statutory Provision (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Expected provision at statutory rate	21.00%	35.00%	35.00%
Difference resulting from:
Tax exempt interest income, net of disallowance	(4.00%)	(6.10%)	(15.60%)
Increase in value of bank owned life insurance assets	(1.10%)	(2.20%)	(4.20%)
Stock-based compensation	(0.20%)	(1.00%)	(1.70%)
Non-deductible merger-related expenses	0.00%	0.00%	1.20%
State income taxes, net of federal benefits	1.60%	1.20%	(1.50%)
Adjustment to deferred tax assets and liabilities for enacted changes in tax laws and rates	(0.50%)	1.70%	0.00%
Changes in valuation allowance	0.10%	0.50%	3.10%
Other	(0.20%)	(0.40%)	0.30%
Provision for income taxes - effective income tax rate	16.70%	28.70%	16.60%