Business Segments - Schedule of Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting Information
Interest and dividend income	$ 317,067	$ 246,972	$ 891,433	$ 644,080
Interest expense	57,842	30,089	143,206	70,736
Net interest income	259,225	216,883	748,227	573,344
Provision for/(reversal of) loan losses	11,097	18,514	26,678	36,718
Other non-interest income/(expense)	34,075	29,150	97,392	83,336
Allocated expenses/(income)	0	0	0	0
Non-interest expense	169,349	200,762	554,522	484,938
Income before income tax expense	131,866	48,450	318,620	201,493
Income tax expense	30,401	18,806	78,410	69,358
Net income	101,465	29,644	240,210	132,135
Total loans, leases, and loans held for sale, net of unearned income	22,386,882	19,936,303	22,386,882	19,936,303
Total assets	30,118,387	27,976,635	30,118,387	27,976,635	$ 27,904,129
Total deposits	23,193,446	21,334,271	23,193,446	21,334,271	$ 21,466,717
Average assets	30,046,231	26,096,921	29,323,445	23,282,834
Operating Segments | IBERIABANK
Segment Reporting Information
Interest and dividend income	315,467	245,166	886,629	638,683
Interest expense	57,842	30,089	143,206	70,736
Net interest income	257,625	215,077	743,423	567,947
Provision for/(reversal of) loan losses	11,099	18,524	26,704	36,816
Other non-interest income/(expense)	33,591	29,163	96,978	83,376
Allocated expenses/(income)	(3,691)	(3,881)	(9,043)	(9,377)
Non-interest expense	154,287	174,306	506,035	412,305
Income before income tax expense	129,521	55,291	316,705	211,579
Income tax expense	29,765	21,076	78,516	72,650
Net income	99,756	34,215	238,189	138,929
Total loans, leases, and loans held for sale, net of unearned income	22,302,679	19,743,749	22,302,679	19,743,749
Total assets	29,974,065	27,738,588	29,974,065	27,738,588
Total deposits	23,179,549	21,333,190	23,179,549	21,333,190
Average assets	29,863,742	25,869,559	29,136,369	23,020,324
Operating Segments | IMC
Segment Reporting Information
Interest and dividend income	1,599	1,805	4,802	5,395
Interest expense	0	0	0	0
Net interest income	1,599	1,805	4,802	5,395
Provision for/(reversal of) loan losses	(2)	(10)	(26)	(98)
Other non-interest income/(expense)	(16)	(9)	(83)	(30)
Allocated expenses/(income)	2,680	2,911	6,714	7,057
Non-interest expense	10,223	22,170	34,105	59,754
Income before income tax expense	1,414	(7,225)	(26)	(11,453)
Income tax expense	386	(2,424)	(7)	(3,840)
Net income	1,028	(4,801)	(19)	(7,613)
Total loans, leases, and loans held for sale, net of unearned income	84,203	192,554	84,203	192,554
Total assets	119,322	217,056	119,322	217,056
Total deposits	13,897	1,081	13,897	1,081
Average assets	158,464	204,920	164,039	239,014
Operating Segments | LTC
Segment Reporting Information
Interest and dividend income	1	1	2	2
Interest expense	0	0	0	0
Net interest income	1	1	2	2
Provision for/(reversal of) loan losses	0	0	0	0
Other non-interest income/(expense)	500	(4)	497	(10)
Allocated expenses/(income)	1,011	970	2,329	2,320
Non-interest expense	4,839	4,286	14,382	12,879
Income before income tax expense	931	384	1,941	1,367
Income tax expense	250	154	(99)	548
Net income	681	230	2,040	819
Total loans, leases, and loans held for sale, net of unearned income	0	0	0	0
Total assets	25,000	20,991	25,000	20,991
Total deposits	0	0	0	0
Average assets	24,025	22,442	23,037	23,496
Mortgage income
Segment Reporting Information
Non-interest Income	12,732	16,050	36,048	49,895
Mortgage income | Operating Segments | IBERIABANK
Segment Reporting Information
Non-interest Income	0	0	0	0
Mortgage income | Operating Segments | IMC
Segment Reporting Information
Non-interest Income	12,732	16,050	36,048	49,895
Mortgage income | Operating Segments | LTC
Segment Reporting Information
Non-interest Income	0	0	0	0
Title revenue
Segment Reporting Information
Non-interest Income	6,280	5,643	18,153	16,574
Title revenue | Operating Segments | IBERIABANK
Segment Reporting Information
Non-interest Income	0	0	0	0
Title revenue | Operating Segments | IMC
Segment Reporting Information
Non-interest Income	0	0	0	0
Title revenue | Operating Segments | LTC
Segment Reporting Information
Non-interest Income	$ 6,280	$ 5,643	$ 18,153	$ 16,574