Allowance for Credit Losses - Summary of Total Non Purchased and Purchased Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 11,089,067		$ 9,693,463
Unearned income	(4,329)		(3,825)
Loans, net of unearned income	11,084,738		9,689,638
Allowance for credit losses	(168,098)	$ (145,387)	(52,162)	$ (50,814)	$ (50,059)	$ (49,026)
Loans, net	10,916,640		9,637,476
Cumulative Effect, Period Of Adoption, Adjusted Balance
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses			(94,646)
Commercial, financial, agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	2,648,062		1,367,972
Loans, net of unearned income	2,648,062		1,367,972
Allowance for credit losses	(38,195)	(30,685)	(10,658)	(10,288)	(9,534)	(8,269)
Lease financing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	87,257		85,700
Loans, net of unearned income	82,928
Allowance for credit losses	(1,832)	(1,812)	(910)
Real estate – construction:
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	773,119		826,483
Loans, net of unearned income	773,119		826,483
Allowance for credit losses	(13,819)	(12,538)	(5,029)	(5,127)	(5,302)	(4,755)
Real estate – construction: | Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	264,525		289,050
Real estate – construction: | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	508,594		537,433
Real estate – 1-4 family mortgage:
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	2,760,394		2,866,613
Loans, net of unearned income	2,760,394		2,866,613
Allowance for credit losses	(32,705)	(29,401)	(9,814)	(9,849)	(9,616)	(10,139)
Real estate – 1-4 family mortgage: | Primary
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	1,762,897		1,781,948
Real estate – 1-4 family mortgage: | Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	537,615		573,540
Real estate – 1-4 family mortgage: | Rental/investment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	306,378		335,100
Real estate – 1-4 family mortgage: | Land development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	153,504		176,025
Real estate – commercial mortgage:
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	4,577,009		4,244,265
Loans, net of unearned income	4,577,009		4,244,265
Allowance for credit losses	(70,582)	(60,061)	(24,990)	(24,039)	(24,302)	(24,492)
Real estate – commercial mortgage: | Land development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	150,366		156,089
Real estate – commercial mortgage: | Owner-occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	1,672,402		1,637,281
Real estate – commercial mortgage: | Non-owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	2,754,241		2,450,895
Installment loans to individuals
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	243,226		302,430
Loans, net of unearned income	243,226		384,305
Allowance for credit losses	$ (10,965)	$ (10,890)	$ (761)	$ (1,511)	$ (1,305)	$ (1,371)