Business Segments - Schedule of Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segment Reporting Information
Interest and dividend income	$ 326,084	$ 270,543
Interest expense	75,600	37,654
Net interest income	250,484	232,889
Provision for/(reversal of) loan losses	13,763	8,211
Other non-interest income (expense)	35,435	29,944
Allocated expenses (income)	0	0
Non-interest expense	158,753	188,071
Income before income tax expense	130,477	81,173
Income tax expense	30,346	17,552
Net income	100,131	63,621
Total loans, leases, and loans held for sale, net of unearned income	23,096,746	21,816,438
Total assets	31,260,189	29,472,637	$ 30,833,015
Total deposits	24,092,062	22,971,192	$ 23,763,431
Average assets	30,833,500	28,132,219
Operating Segments | IBERIABANK
Segment Reporting Information
Interest and dividend income	324,644	268,775
Interest expense	75,600	37,654
Net interest income	249,044	231,121
Provision for/(reversal of) loan losses	13,823	8,216
Other non-interest income (expense)	35,463	29,906
Allocated expenses (income)	(2,033)	(1,479)
Non-interest expense	143,755	171,591
Income before income tax expense	128,962	82,699
Income tax expense	29,975	18,540
Net income	98,987	64,159
Total loans, leases, and loans held for sale, net of unearned income	22,944,800	21,652,223
Total assets	31,044,209	29,243,983
Total deposits	24,078,698	22,959,061
Average assets	30,660,806	27,924,587
Operating Segments | IMC
Segment Reporting Information
Interest and dividend income	1,439	1,767
Interest expense	0	0
Net interest income	1,439	1,767
Provision for/(reversal of) loan losses	(60)	(5)
Other non-interest income (expense)	(12)	38
Allocated expenses (income)	1,500	1,166
Non-interest expense	10,541	11,916
Income before income tax expense	1,295	(1,677)
Income tax expense	307	(410)
Net income	988	(1,267)
Total loans, leases, and loans held for sale, net of unearned income	151,946	164,215
Total assets	191,254	205,497
Total deposits	13,364	12,131
Average assets	148,174	185,742
Operating Segments | LTC
Segment Reporting Information
Interest and dividend income	1	1
Interest expense	0	0
Net interest income	1	1
Provision for/(reversal of) loan losses	0	0
Other non-interest income (expense)	(16)	0
Allocated expenses (income)	533	313
Non-interest expense	4,457	4,564
Income before income tax expense	220	151
Income tax expense	64	(578)
Net income	156	729
Total loans, leases, and loans held for sale, net of unearned income	0	0
Total assets	24,726	23,157
Total deposits	0	0
Average assets	24,520	21,890
Mortgage income
Segment Reporting Information
Non-interest income	11,849	9,595
Mortgage income | Operating Segments | IBERIABANK
Segment Reporting Information
Non-interest income	0	0
Mortgage income | Operating Segments | IMC
Segment Reporting Information
Non-interest income	11,849	9,595
Mortgage income | Operating Segments | LTC
Segment Reporting Information
Non-interest income	0	0
Title income
Segment Reporting Information
Non-interest income	5,225	5,027
Title income | Operating Segments | IBERIABANK
Segment Reporting Information
Non-interest income	0	0
Title income | Operating Segments | IMC
Segment Reporting Information
Non-interest income	0	0
Title income | Operating Segments | LTC
Segment Reporting Information
Non-interest income	$ 5,225	$ 5,027