Loans Held for Investment and Allowance for Credit Losses on Loans - Summary of Amounts Included in Nonaccrual Loans Segregated by Portfolio Segment (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans, Total	$ 45,181	$ 17,965
Real Estate [Member] | Commercial Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans, Total	2,503
Real Estate [Member] | Commercial Real Estate Non-Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans, Total	465
Real Estate [Member] | Construction and Development Less than 60 Months [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans, Total	155
Real Estate [Member] | Construction Residential Real Estate Less than 60 Months [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans, Total	380
Real Estate [Member] | Residential Real Estate First Lien [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans, Total	3,657
Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans, Total		2,275
Real Estate [Member] | Non-residential Real Estate Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans, Total		1,815
Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans, Total		1,206
Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans, Total	838	3,060
Real Estate [Member] | Farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans, Total	2,687
Commercial and Financial [Member] | Non-consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans, Total		2,915
Commercial and Agricultural Non-Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans, Total	32,530
Consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans, Total	174	264
Other Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans, Total	1,522	1,083
Acquired Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans, Total		4,496
Pegasus Bank [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans, Total	$ 270	$ 851