Business Segment Information - Amounts of Consolidated Revenue, Expense, Tax and Assets (Details) - USD ($) $ in Millions		12 Months Ended
	Jul. 01, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net interest income		$ 1,662	$ 1,210	$ 1,220
Provision for credit losses		503	45	8
Noninterest income		1,492	654	723
Noninterest expense		1,718	1,233	1,221
Income before income taxes		933	586	714
Income tax expense		76	134	157
Net income		857	452	557
Average assets		64,346	41,744	40,225
Depreciation and amortization		46	65	59
Expenditures for long-lived assets		379	49	38
Purchase accounting gain		533	0	0
Expenses associated with rebranding initiatives			21
Charitable foundation contributions		41	11
IBERIABANK (IBKC)
Segment Reporting Information [Line Items]
Purchase accounting gain	$ 533	533
Regional Banking
Segment Reporting Information [Line Items]
Net interest income		1,307	773	815
Provision for credit losses		392	24	4
Noninterest income		343	289	264
Noninterest expense		900	626	707
Income before income taxes		358	412	368
Income tax expense		77	94	82
Net income		281	318	286
Average assets		31,802	18,252	17,263
Depreciation and amortization		(46)	22	18
Expenditures for long-lived assets		283	29	36
Purchase accounting gain		0
Specialty Banking
Segment Reporting Information [Line Items]
Net interest income		583	444	417
Provision for credit losses		117	37	15
Noninterest income		576	318	210
Noninterest expense		491	351	302
Income before income taxes		551	374	310
Income tax expense		134	93	76
Net income		417	281	234
Average assets		19,713	15,508	14,420
Depreciation and amortization		3	14	19
Expenditures for long-lived assets		6	4	2
Purchase accounting gain		0
Corporate
Segment Reporting Information [Line Items]
Net interest income		(228)	(7)	(12)
Provision for credit losses		(6)	(16)	(11)
Noninterest income		573	47	249
Noninterest expense		327	256	212
Income before income taxes		24	(200)	36
Income tax expense		(135)	(53)	(1)
Net income		159	(147)	37
Average assets		12,831	7,984	8,542
Depreciation and amortization		89	29	22
Expenditures for long-lived assets		90	$ 16	0
Purchase accounting gain		$ 533
Visa Class B Shares
Segment Reporting Information [Line Items]
Gain on sale of investments				$ 213