Allowance for Credit Losses - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan and lease losses		$ 963	$ 200	$ 180	$ 189
IBERIABANK (IBKC)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit loss, period increase (decrease)	$ 284
COVID-19 Deferrals
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan and lease losses		$ 1