Business Segments (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) Segment	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information
Number of business segments | Segment			3
Segment Reporting Information, Revenue for Reportable Segment [Abstract]
Interest and dividend income	$ 333,178	$ 317,067	$ 995,229	$ 891,433
Interest expense	83,845	57,842	240,073	143,206
Net interest income	249,333	259,225	755,156	748,227
Other Income and Expenses [Abstract]
Provision for/(reversal of) loan losses	8,986	11,384	33,504	27,290
Other non-interest income (expense)	39,072	34,078	107,993	97,395
Allocated expenses (income)	0	0	0	0
Non-interest expense	172,662	169,062	501,033	553,910
Income before income tax expense	131,359	131,866	395,627	318,620
Income tax expense	31,509	30,401	94,048	78,410
Net income	99,850	101,465	301,579	240,210
Total loans, leases, and loans held for sale, net of unearned income	23,931,813	22,386,882	23,931,813	22,386,882
Total assets	31,734,598	30,118,387	31,734,598	30,118,387	$ 30,833,015
Deposits	24,977,285	23,193,446	24,977,285	23,193,446	$ 23,763,431
Average assets	31,554,291	30,046,231	31,222,511	29,323,445
Title revenue
Other Income and Expenses [Abstract]
Non-interest income	7,170	6,280	19,290	18,153
Mortgage income
Other Income and Expenses [Abstract]
Non-interest income	17,432	12,729	47,725	36,045
Operating Segments | IBERIABANK
Segment Reporting Information, Revenue for Reportable Segment [Abstract]
Interest and dividend income	330,896	315,467	989,595	886,629
Interest expense	83,845	57,842	240,073	143,206
Net interest income	247,051	257,625	749,522	743,423
Other Income and Expenses [Abstract]
Provision for/(reversal of) loan losses	8,974	11,386	33,539	27,316
Other non-interest income (expense)	35,881	33,594	104,840	96,981
Allocated expenses (income)	(4,068)	(3,691)	(10,515)	(9,043)
Non-interest expense	152,988	154,000	447,882	505,423
Income before income tax expense	125,038	129,524	383,456	316,708
Income tax expense	29,938	29,765	91,047	78,516
Net income	95,100	99,759	292,409	238,192
Total loans, leases, and loans held for sale, net of unearned income	23,650,057	22,302,679	23,650,057	22,302,679
Total assets	31,386,350	29,974,065	31,386,350	29,974,065
Deposits	24,946,708	23,179,549	24,946,708	23,179,549
Average assets	31,256,216	29,863,742	30,981,906	29,136,369
Operating Segments | IMC
Segment Reporting Information, Revenue for Reportable Segment [Abstract]
Interest and dividend income	2,282	1,599	5,633	4,802
Interest expense	0	0	0	0
Net interest income	2,282	1,599	5,633	4,802
Other Income and Expenses [Abstract]
Provision for/(reversal of) loan losses	12	(2)	(35)	(26)
Other non-interest income (expense)	3,194	(16)	3,172	(83)
Allocated expenses (income)	2,991	2,680	7,741	6,714
Non-interest expense	14,676	10,223	38,807	34,105
Income before income tax expense	5,229	1,411	10,017	(29)
Income tax expense	1,282	386	2,425	(7)
Net income	3,947	1,025	7,592	(22)
Total loans, leases, and loans held for sale, net of unearned income	281,756	84,203	281,756	84,203
Total assets	320,994	119,322	320,994	119,322
Deposits	30,577	13,897	30,577	13,897
Average assets	271,817	158,464	215,306	164,039
Operating Segments | LTC
Segment Reporting Information, Revenue for Reportable Segment [Abstract]
Interest and dividend income	0	1	1	2
Interest expense	0	0	0	0
Net interest income	0	1	1	2
Other Income and Expenses [Abstract]
Provision for/(reversal of) loan losses	0	0	0	0
Other non-interest income (expense)	(3)	500	(19)	497
Allocated expenses (income)	1,077	1,011	2,774	2,329
Non-interest expense	4,998	4,839	14,344	14,382
Income before income tax expense	1,092	931	2,154	1,941
Income tax expense	289	250	576	(99)
Net income	803	681	1,578	2,040
Total loans, leases, and loans held for sale, net of unearned income	0	0	0	0
Total assets	27,254	25,000	27,254	25,000
Deposits	0	0	0	0
Average assets	26,258	24,025	25,299	23,037
Operating Segments | Title revenue | IBERIABANK
Other Income and Expenses [Abstract]
Non-interest income	0	0	0	0
Operating Segments | Title revenue | IMC
Other Income and Expenses [Abstract]
Non-interest income	0	0	0	0
Operating Segments | Title revenue | LTC
Other Income and Expenses [Abstract]
Non-interest income	7,170	6,280	19,290	18,153
Operating Segments | Mortgage income | IBERIABANK
Other Income and Expenses [Abstract]
Non-interest income	0	0	0	0
Operating Segments | Mortgage income | IMC
Other Income and Expenses [Abstract]
Non-interest income	17,432	12,729	47,725	36,045
Operating Segments | Mortgage income | LTC
Other Income and Expenses [Abstract]
Non-interest income	$ 0	$ 0	$ 0	$ 0