Business Segments - Schedule of Segment Reporting Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Interest income	$ 138,585	$ 114,232
Interest expense	12,781	9,824
Net interest income	125,804	104,408
Provision for loan losses	5,345	2,103
Mortgage income	18,023	10,133
Title revenue	4,629	4,167
Other non-interest income	26,247	21,381
Non-interest expenses	133,153	107,234
Income before income tax expense	36,205	30,752
Income tax expense	11,079	8,416
Net income	25,126	22,336
Total loans and loans held for sale	13,088,505	9,772,772
Total assets	18,054,671	13,550,232	15,757,904
Total deposits	14,668,073	10,898,863	12,520,525
Average assets	15,939,485	13,362,237
IBERIABANK [Member]
Segment Reporting Information [Line Items]
Interest income	136,830	113,149
Interest expense	12,290	9,603
Net interest income	124,540	103,546
Provision for loan losses	5,345	2,096
Mortgage income	(1)	(3)
Other non-interest income	26,251	21,392
Allocated expenses	(4,847)	(3,130)
Non-interest expenses	116,005	93,046
Income before income tax expense	34,287	32,923
Income tax expense	10,313	9,257
Net income	23,974	23,666
Total loans and loans held for sale	12,869,096	9,620,359
Total assets	17,775,541	13,349,465
Total deposits	14,663,184	10,896,508
Average assets	15,732,790	13,198,208
IMC [Member]
Segment Reporting Information [Line Items]
Interest income	1,754	1,082
Interest expense	491	221
Net interest income	1,263	861
Provision for loan losses		7
Mortgage income	18,024	10,136
Other non-interest income	(2)	(11)
Allocated expenses	3,528	2,191
Non-interest expenses	12,916	10,299
Income before income tax expense	2,841	(1,511)
Income tax expense	1,122	(588)
Net income	1,719	(923)
Total loans and loans held for sale	219,409	152,413
Total assets	254,401	176,106
Total deposits	4,889	2,355
Average assets	181,942	139,057
LTC [Member]
Segment Reporting Information [Line Items]
Interest income	1	1
Net interest income	1	1
Title revenue	4,629	4,167
Other non-interest income	(2)
Allocated expenses	1,319	939
Non-interest expenses	4,232	3,889
Income before income tax expense	(923)	(660)
Income tax expense	(356)	(253)
Net income	(567)	(407)
Total assets	24,729	24,661
Average assets	$ 24,753	$ 24,972