Business Segment Information - Disaggregation of Revenue (Details) - USD ($) $ in Millions		12 Months Ended
	Jul. 01, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Fixed income		$ 423	$ 279	$ 168
Deposit transactions and cash management		148	132	133
Mortgage banking and title income		129	10	11
Brokerage, management fees and commissions		66	55	55
Trust services and investment management		39	30	30
Bankcard income		37	28	29
Securities gains (losses), net		(6)	0	213
Purchase accounting gain		533	0	0
Other income		123	120	84
Total noninterest income		1,492	654	723
IBERIABANK (IBKC)
Disaggregation of Revenue [Line Items]
Purchase accounting gain	$ 533	533
Visa Class B Shares
Disaggregation of Revenue [Line Items]
Gain on sale of investments				213
Underwriting, Portfolio Advisory, and Other Noninterest Income
Disaggregation of Revenue [Line Items]
Revenue from contract with customer		39	34	29
Regional Banking
Disaggregation of Revenue [Line Items]
Fixed income		1	0	0
Deposit transactions and cash management		131	114	110
Mortgage banking and title income		0	0	0
Brokerage, management fees and commissions		66	55	55
Trust services and investment management		39	30	30
Bankcard income		34	26	28
Securities gains (losses), net		0		0
Purchase accounting gain		0
Other income		72	64	41
Total noninterest income		343	289	264
Specialty Banking
Disaggregation of Revenue [Line Items]
Fixed income		422	278	164
Deposit transactions and cash management		11	11	17
Mortgage banking and title income		128	8	8
Brokerage, management fees and commissions		0	0	0
Trust services and investment management		0	0	0
Bankcard income		2	2	1
Securities gains (losses), net		0		0
Purchase accounting gain		0
Other income		13	19	20
Total noninterest income		576	318	210
Corporate
Disaggregation of Revenue [Line Items]
Fixed income		0	1	4
Deposit transactions and cash management		6	7	6
Mortgage banking and title income		1	2	3
Brokerage, management fees and commissions		0	0	0
Trust services and investment management		0	0	0
Bankcard income		1	0	0
Securities gains (losses), net		(6)		213
Purchase accounting gain		533
Other income		38	37	23
Total noninterest income		$ 573	47	249
Non Strategic
Disaggregation of Revenue [Line Items]
Gain on sale of TRUPs loans			$ 1	$ 4