Business Segments - Schedule of Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Segment Reporting Information
Interest and dividend income	$ 192,533	$ 176,936
Interest expense	19,715	15,533
Net interest income	172,818	161,403
Provision for loan losses	6,154	14,905
Mortgage income	14,115	19,940
Service charges on deposit accounts	11,153	10,951
Title revenue	4,741	4,745
Other non-interest income	17,337	20,209
Allocated expenses	0	0
Non-interest expense	141,018	137,452
Income before income tax expense	72,992	64,891
Income tax expense	22,519	22,122
Net income	50,473	42,769
Total loans and loans held for sale, net of unearned income	15,254,535	14,643,789
Total assets	22,008,479	20,092,563	$ 21,659,190
Total deposits	17,312,265	16,260,566	$ 17,408,283
Average assets	21,861,501	19,661,311
Operating Segments | IBERIABANK
Segment Reporting Information
Interest and dividend income	190,823	175,324
Interest expense	19,715	14,654
Net interest income	171,108	160,670
Provision for loan losses	6,158	14,905
Mortgage income	0	0
Service charges on deposit accounts	11,153	10,951
Title revenue	0	0
Other non-interest income	17,353	20,210
Allocated expenses	(2,174)	(2,669)
Non-interest expense	121,651	120,027
Income before income tax expense	73,979	59,568
Income tax expense	22,829	20,001
Net income	51,150	39,567
Total loans and loans held for sale, net of unearned income	15,072,661	14,428,613
Total assets	21,774,586	19,774,092
Total deposits	17,306,328	16,256,147
Average assets	21,550,930	19,381,718
Operating Segments | IMC
Segment Reporting Information
Interest and dividend income	1,709	1,611
Interest expense	0	879
Net interest income	1,709	732
Provision for loan losses	(4)	0
Mortgage income	14,115	19,940
Service charges on deposit accounts	0	0
Title revenue	0	0
Other non-interest income	(10)	(1)
Allocated expenses	1,656	2,050
Non-interest expense	15,167	13,198
Income before income tax expense	(1,005)	5,423
Income tax expense	(322)	2,153
Net income	(683)	3,270
Total loans and loans held for sale, net of unearned income	181,874	215,176
Total assets	209,911	291,893
Total deposits	5,937	4,419
Average assets	286,694	252,281
Operating Segments | LTC
Segment Reporting Information
Interest and dividend income	1	1
Interest expense	0	0
Net interest income	1	1
Provision for loan losses	0	0
Mortgage income	0	0
Service charges on deposit accounts	0	0
Title revenue	4,741	4,745
Other non-interest income	(6)	0
Allocated expenses	518	619
Non-interest expense	4,200	4,227
Income before income tax expense	18	(100)
Income tax expense	12	(32)
Net income	6	(68)
Total loans and loans held for sale, net of unearned income	0	0
Total assets	23,982	26,578
Total deposits	0	0
Average assets	$ 23,877	$ 27,312