Business Segments - Schedule of Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment Reporting Information
Interest and dividend income	$ 270,543	$ 192,533
Interest expense	37,654	19,715
Net interest income	232,889	172,818
Provision for loan and lease losses	7,986	6,154
Mortgage income	9,595	14,115
Service charges on deposit accounts	12,908	11,153
Title revenue	5,027	4,741
Other non-interest income	29,944	26,268
Allocated expenses	0	0
Non-interest expense	188,296	138,796
Income before income tax expense	81,173	72,992
Income tax expense	17,552	22,519
Net income	63,621	50,473
Total loans, leases, and loans held for sale, net of unearned income	21,816,438	15,254,535
Total assets	29,472,637	22,008,479	$ 27,904,129
Total deposits	22,971,192	17,312,265	$ 21,466,717
Average assets	28,132,219	21,861,501
Operating Segments | IBERIABANK
Segment Reporting Information
Interest and dividend income	268,775	190,823
Interest expense	37,654	19,715
Net interest income	231,121	171,108
Provision for loan and lease losses	7,991	6,158
Mortgage income	0	0
Title revenue	0	0
Other non-interest income	29,906	26,284
Allocated expenses	(1,479)	(2,174)
Non-interest expense	171,816	119,429
Income before income tax expense	82,699	73,979
Income tax expense	18,540	22,829
Net income	64,159	51,150
Total loans, leases, and loans held for sale, net of unearned income	21,652,223	15,072,661
Total assets	29,243,983	21,774,586
Total deposits	22,959,061	17,306,328
Average assets	27,924,587	21,550,930
Operating Segments | IMC
Segment Reporting Information
Interest and dividend income	1,767	1,709
Interest expense	0	0
Net interest income	1,767	1,709
Provision for loan and lease losses	(5)	(4)
Mortgage income	9,595	14,115
Title revenue	0	0
Other non-interest income	38	(10)
Allocated expenses	1,166	1,656
Non-interest expense	11,916	15,167
Income before income tax expense	(1,677)	(1,005)
Income tax expense	(410)	(322)
Net income	(1,267)	(683)
Total loans, leases, and loans held for sale, net of unearned income	164,215	181,874
Total assets	205,497	209,911
Total deposits	12,131	5,937
Average assets	185,742	286,694
Operating Segments | LTC
Segment Reporting Information
Interest and dividend income	1	1
Interest expense	0	0
Net interest income	1	1
Provision for loan and lease losses	0	0
Mortgage income	0	0
Title revenue	5,027	4,741
Other non-interest income	0	(6)
Allocated expenses	313	518
Non-interest expense	4,564	4,200
Income before income tax expense	151	18
Income tax expense	(578)	12
Net income	729	6
Total loans, leases, and loans held for sale, net of unearned income	0	0
Total assets	23,157	23,982
Total deposits	0	0
Average assets	$ 21,890	$ 23,877