Credit Quality - Schedule of Troubled Debt Restructurings, Segregated by Class of Loans (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Contract	Dec. 31, 2016 USD ($) Contract
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	15	9
Pre-Modification Outstanding Recorded Investment	$ 39,000	$ 5,393
Post-Modification Outstanding Recorded Investment	$ 36,121	$ 4,791
Construction & Land Development [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	1	0
Pre-Modification Outstanding Recorded Investment	$ 1,456	$ 0
Post-Modification Outstanding Recorded Investment	$ 1,383	$ 0
Commercial Real Estate [Member] | Owner-Occupied [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	1	1
Pre-Modification Outstanding Recorded Investment	$ 5,333	$ 1,190
Post-Modification Outstanding Recorded Investment	$ 5,327	$ 1,178
Commercial Real Estate [Member] | Nonowner-Occupied [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	0	0
Pre-Modification Outstanding Recorded Investment	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	$ 0	$ 0
Other Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	13	7
Pre-Modification Outstanding Recorded Investment	$ 32,211	$ 2,803
Post-Modification Outstanding Recorded Investment	$ 29,411	$ 2,213
Residential Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	0	1
Pre-Modification Outstanding Recorded Investment	$ 0	$ 1,400
Post-Modification Outstanding Recorded Investment	$ 0	$ 1,400
Consumer [Member] | Bankcard [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	0	0
Pre-Modification Outstanding Recorded Investment	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	$ 0	$ 0
Consumer [Member] | Other Consumer [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	0	0
Pre-Modification Outstanding Recorded Investment	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	$ 0	$ 0