Business Segments - Schedule of Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Segment Reporting Information
Interest and dividend income	$ 178,694	$ 160,545	$ 355,630	$ 299,130
Interest expense	15,941	14,868	31,474	27,649
Net interest income	162,753	145,677	324,156	271,481
Provision for loan losses	11,866	8,790	26,771	14,135
Mortgage income	25,991	25,246	45,931	43,269
Service charges on deposit accounts	10,940	10,162	21,891	19,424
Title revenue	6,135	6,146	10,880	10,775
Other non-interest income	21,851	19,959	42,060	36,944
Allocated expenses	0	0	0	0
Non-interest expense	139,504	153,209	276,956	286,362
Income before income tax expense	76,300	45,191	141,191	81,396
Income tax expense	25,490	14,355	47,612	25,434
Net Income	50,810	30,836	93,579	55,962
Total loans and loans held for sale, net of unearned income	14,952,214	14,171,328	14,952,214	14,171,328
Total assets	20,160,855	19,238,928	20,160,855	19,238,928	$ 19,504,068
Total deposits	15,862,027	16,119,541	15,862,027	16,119,541	$ 16,178,748
Average assets	20,003,917	18,445,286	19,832,614	17,208,321
Operating Segments | IBERIABANK
Segment Reporting Information
Interest and dividend income	176,564	158,940	351,888	295,770
Interest expense	14,782	14,011	29,436	26,301
Net interest income	161,782	144,929	322,452	269,469
Provision for loan losses	11,866	8,790	26,771	14,135
Mortgage income	7	568	6	567
Service charges on deposit accounts	10,940	10,162	21,891	19,424
Title revenue	0	0	0	0
Other non-interest income	21,843	19,965	42,053	36,954
Allocated expenses	(3,885)	(3,238)	(6,554)	(8,085)
Non-interest expense	120,268	133,034	240,295	249,039
Income before income tax expense	66,323	37,038	125,890	71,325
Income tax expense	21,558	11,129	41,559	21,442
Net Income	44,765	25,909	84,331	49,883
Total loans and loans held for sale, net of unearned income	14,702,843	13,928,039	14,702,843	13,928,039
Total assets	19,807,507	18,924,178	19,807,507	18,924,178
Total deposits	15,855,908	16,112,387	15,855,908	16,112,387
Average assets	19,668,456	18,168,782	19,525,087	16,966,529
Operating Segments | IMC
Segment Reporting Information
Interest and dividend income	2,130	1,605	3,741	3,359
Interest expense	1,159	857	2,038	1,348
Net interest income	971	748	1,703	2,011
Provision for loan losses	0	0	0	0
Mortgage income	25,984	24,678	45,925	42,702
Service charges on deposit accounts	0	0	0	0
Title revenue	0	0	0	0
Other non-interest income	8	(1)	7	(3)
Allocated expenses	2,947	2,464	4,997	5,992
Non-interest expense	14,820	15,738	28,018	28,654
Income before income tax expense	9,196	7,223	14,620	10,064
Income tax expense	3,625	2,858	5,778	3,980
Net Income	5,571	4,365	8,842	6,084
Total loans and loans held for sale, net of unearned income	249,371	243,289	249,371	243,289
Total assets	326,397	289,450	326,397	289,450
Total deposits	6,119	7,154	6,119	7,154
Average assets	308,647	251,475	280,464	216,900
Operating Segments | LTC
Segment Reporting Information
Interest and dividend income	0	0	1	1
Interest expense	0	0	0	0
Net interest income	0	0	1	1
Provision for loan losses	0	0	0	0
Mortgage income	0	0	0	0
Service charges on deposit accounts	0	0	0	0
Title revenue	6,135	6,146	10,880	10,775
Other non-interest income	0	(5)	0	(7)
Allocated expenses	938	774	1,557	2,093
Non-interest expense	4,416	4,437	8,643	8,669
Income before income tax expense	781	930	681	7
Income tax expense	307	368	275	12
Net Income	474	562	406	(5)
Total loans and loans held for sale, net of unearned income	0	0	0	0
Total assets	26,951	25,300	26,951	25,300
Total deposits	0	0	0	0
Average assets	$ 26,814	$ 25,029	$ 27,063	$ 24,892