Taxes on Income - Schedule of Effective Income Tax Rate Reconciliation (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Statutory federal income tax rate	35.00%	35.00%	35.00%
State tax, net of federal tax benefit	2.70%	3.10%	2.90%
Adjustment to net deferred tax asset for enacted changes in tax laws and rates	13.40%	0.00%	0.00%
Nondeductible acquisition costs	0.00%	0.20%	0.70%
Tax-exempt interest	(1.90%)	(2.10%)	(1.90%)
Bank-owned life insurance income	(0.50%)	(0.30%)	(0.30%)
Excess tax benefits from share-based compensation	(2.00%)	(1.40%)	0.00%
Surrender of bank-owned life insurance policies	7.30%	0.00%	0.00%
Federal tax credits, net of amortization	(0.30%)	(0.50%)	(0.50%)
Other	0.00%	0.00%	0.20%
Effective tax rate	53.70%	34.00%	36.10%