Loans Held for Investment and Allowance for Credit Losses on Loans - Summary of Loans Held for Investment by Portfolio Segment (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans (net of unearned interest)	$ 6,394,506	$ 5,662,143
Real Estate [Member] | Commercial Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans (net of unearned interest)	641,987	621,188
Real Estate [Member] | Commercial Real Estate Non-Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans (net of unearned interest)	971,158	851,200
Real Estate [Member] | Construction and Development Less than 60 Months [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans (net of unearned interest)	229,615	287,138
Real Estate [Member] | Construction Residential Real Estate Less than 60 Months [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans (net of unearned interest)	206,195	189,480
Real Estate [Member] | Residential Real Estate First Lien [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans (net of unearned interest)	853,316	834,849
Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans (net of unearned interest)	168,081	187,647
Real Estate [Member] | Farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans (net of unearned interest)	252,958	246,988
Commercial and Agricultural Non-Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans (net of unearned interest)	1,159,810	1,315,005
Consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans (net of unearned interest)	355,405	359,529
Oil and Gas [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans (net of unearned interest)	179,355	184,399
Other Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans (net of unearned interest)	822,078	154,015
Pegasus Bank [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans (net of unearned interest)	554,548	$ 430,705
Pegasus Bank [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans (net of unearned interest)	$ 272,000