Loans and Leases - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Credit Quality Indicator [Line Items]
Nonaccrual interest income recognized			$ 4
Charge-offs			5,899	$ 3,463	$ 15,651
Loans and leases held for investment			5,306,841	4,386,836
Recoveries			1,251	912	3,150
Minimum [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amount of loan relationship balance, loans reviewed annually			1,000
Maximum [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amount of loan relationship balance, loans reviewed on a performance basis			1,000
Commercial, Financial and Agricultural [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Charge-offs			1,884	1,965	14,655
Loans and leases held for investment			892,665	947,029
Recoveries			$ 745	$ 367	2,140
Loan Participations [Member] | Commercial, Financial and Agricultural [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Charge-offs		$ 12,700
Loans and leases held for investment	$ 13,000				13,000
Total participating interest from all parties	80,000				80,000
Recoveries	$ 1,800
Allowance for loan and lease losses write-offs, net					$ 10,900