Purchased Loans - Loans Purchased with Deteriorated Credit Quality (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance of restructured loans	$ 10,916,640	$ 9,637,476
Purchased with deteriorated credit quality | Purchased loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance of restructured loans		172,264
Purchased with deteriorated credit quality | Purchased loans | Commercial, financial, agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance of restructured loans		29,538
Purchased with deteriorated credit quality | Purchased loans | Real estate – construction:
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance of restructured loans		863
Purchased with deteriorated credit quality | Purchased loans | Real estate – 1-4 family mortgage:
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance of restructured loans		35,543
Purchased with deteriorated credit quality | Purchased loans | Real estate – commercial mortgage:
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance of restructured loans		103,133
Purchased with deteriorated credit quality | Purchased loans | Installment loans to individuals
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance of restructured loans		$ 3,187