Income Taxes - Income Tax Provision Differences from Expected Statutory Provision (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Expected provision at statutory rate	35.00%	35.00%	35.00%
Difference resulting from:
Tax exempt interest income, net of disallowance	(6.10%)	(15.60%)	(9.50%)
Increase in value of bank owned life insurance assets	(2.20%)	(4.20%)	(1.20%)
Stock-based compensation	(1.00%)	(1.70%)	0.50%
Non-deductible merger-related expenses	0.00%	1.20%	0.40%
State income taxes, net of federal benefits	1.20%	(1.50%)	0.90%
Adjustment to deferred tax assets and liabilities for enacted changes in tax laws and rates	1.70%	0.00%	0.00%
Changes in valuation allowance	0.50%	3.10%	0.40%
Other	(0.40%)	0.30%	(0.10%)
Provision for income taxes - effective income tax rate	28.70%	16.60%	26.40%