Business Segments - Schedule of Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment Reporting Information
Interest and dividend income	$ 204,575	$ 178,694	$ 397,108	$ 355,630
Interest expense	20,932	15,941	40,647	31,474
Net interest income	183,643	162,753	356,461	324,156
Provision for loan losses	12,050	11,866	18,204	26,771
Mortgage income	19,730	25,991	33,845	45,931
Service charges on deposit accounts	11,410	10,940	22,563	21,891
Title revenue	6,190	6,135	10,931	10,880
Other non-interest income	18,636	21,851	35,973	42,060
Allocated expenses	0	0	0	0
Non-interest expense	147,508	139,504	288,526	276,956
Income before income tax expense	80,051	76,300	153,043	141,191
Income tax expense	28,033	25,490	50,552	47,612
Net income	52,018	50,810	102,491	93,579
Total loans and loans held for sale, net of unearned income	15,696,975	14,952,214	15,696,975	14,952,214
Total assets	21,790,727	20,160,855	21,790,727	20,160,855	$ 21,659,190
Total deposits	16,853,116	15,862,027	16,853,116	15,862,027	$ 17,408,283
Average assets	21,843,537	20,003,917	21,852,469	19,832,614
Operating Segments | IBERIABANK
Segment Reporting Information
Interest and dividend income	202,694	176,564	393,517	351,888
Interest expense	20,932	14,782	40,647	29,436
Net interest income	181,762	161,782	352,870	322,452
Provision for loan losses	12,134	11,866	18,292	26,771
Mortgage income	0	7	0	6
Service charges on deposit accounts	11,410	10,940	22,563	21,891
Title revenue	0	0	0	0
Other non-interest income	18,647	21,843	36,000	42,053
Allocated expenses	(3,322)	(3,885)	(5,496)	(6,554)
Non-interest expense	120,698	120,268	242,349	240,295
Income before income tax expense	82,309	66,323	156,288	125,890
Income tax expense	28,745	21,558	51,574	41,559
Net income	53,564	44,765	104,714	84,331
Total loans and loans held for sale, net of unearned income	15,504,171	14,702,843	15,504,171	14,702,843
Total assets	21,549,557	19,807,507	21,549,557	19,807,507
Total deposits	16,852,620	15,855,908	16,852,620	15,855,908
Average assets	21,593,026	19,668,456	21,572,094	19,525,087
Operating Segments | IMC
Segment Reporting Information
Interest and dividend income	1,881	2,130	3,590	3,741
Interest expense	0	1,159	0	2,038
Net interest income	1,881	971	3,590	1,703
Provision for loan losses	(84)	0	(88)	0
Mortgage income	19,730	25,984	33,845	45,925
Service charges on deposit accounts	0	0	0	0
Title revenue	0	0	0	0
Other non-interest income	(11)	8	(21)	7
Allocated expenses	2,490	2,947	4,146	4,997
Non-interest expense	22,417	14,820	37,584	28,018
Income before income tax expense	(3,223)	9,196	(4,228)	14,620
Income tax expense	(1,094)	3,625	(1,416)	5,778
Net income	(2,129)	5,571	(2,812)	8,842
Total loans and loans held for sale, net of unearned income	192,804	249,371	192,804	249,371
Total assets	215,862	326,397	215,862	326,397
Total deposits	496	6,119	496	6,119
Average assets	226,326	308,647	256,343	280,464
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	6,190	6,135	10,931	10,880
Other non-interest income	0	0	(6)	0
Allocated expenses	832	938	1,350	1,557
Non-interest expense	4,393	4,416	8,593	8,643
Income before income tax expense	965	781	983	681
Income tax expense	382	307	394	275
Net income	583	474	589	406
Total loans and loans held for sale, net of unearned income	0	0	0	0
Total assets	25,308	26,951	25,308	26,951
Total deposits	0	0	0	0
Average assets	$ 24,185	$ 26,814	$ 24,032	$ 27,063