Business Segments - Schedule of Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Segment Reporting Information
Interest and dividend income	$ 303,823	$ 204,575	$ 574,366	$ 397,108
Interest expense	47,710	20,932	85,364	40,647
Net interest income	256,113	183,643	489,002	356,461
Provision for/(reversal of) loan losses	7,595	12,050	15,581	18,204
Mortgage income	13,721	19,730	23,316	33,845
Title revenue	6,846	6,190	11,873	10,931
Other non-interest income/(loss)	33,373	27,918	63,317	54,186
Allocated expenses	0	0	0	0
Non-interest expense	196,877	145,380	385,173	284,176
Income before income tax expense	105,581	80,051	186,754	153,043
Income tax expense	30,457	28,033	48,009	50,552
Net income	75,124	52,018	138,745	102,491
Total loans, leases, and loans held for sale, net of unearned income	22,154,626	15,696,975	22,154,626	15,696,975
Total assets	30,126,162	21,790,727	30,126,162	21,790,727	$ 27,904,129
Total deposits	23,430,458	16,853,116	23,430,458	16,853,116	$ 21,466,717
Average assets	29,770,852	21,843,537	28,956,062	21,852,469
Operating Segments | IBERIABANK
Segment Reporting Information
Interest and dividend income	302,387	202,694	571,162	393,517
Interest expense	47,710	20,932	85,364	40,647
Net interest income	254,677	181,762	485,798	352,870
Provision for/(reversal of) loan losses	7,614	12,134	15,605	18,292
Mortgage income	0	0	0	0
Title revenue	0	0	0	0
Other non-interest income/(loss)	33,481	27,929	63,387	54,213
Allocated expenses	(3,873)	(3,322)	(5,352)	(5,496)
Non-interest expense	179,932	118,570	351,748	237,999
Income before income tax expense	104,485	82,309	187,184	156,288
Income tax expense	30,211	28,745	48,751	51,574
Net income	74,274	53,564	138,433	104,714
Total loans, leases, and loans held for sale, net of unearned income	22,026,914	15,504,171	22,026,914	15,504,171
Total assets	29,928,360	21,549,557	29,928,360	21,549,557
Total deposits	23,419,382	16,852,620	23,419,382	16,852,620
Average assets	29,599,468	21,593,026	28,766,655	21,572,094
Operating Segments | IMC
Segment Reporting Information
Interest and dividend income	1,436	1,881	3,203	3,590
Interest expense	0	0	0	0
Net interest income	1,436	1,881	3,203	3,590
Provision for/(reversal of) loan losses	(19)	(84)	(24)	(88)
Mortgage income	13,721	19,730	23,316	33,845
Title revenue	0	0	0	0
Other non-interest income/(loss)	(105)	(11)	(67)	(21)
Allocated expenses	2,868	2,490	4,034	4,146
Non-interest expense	11,966	22,417	23,882	37,584
Income before income tax expense	237	(3,223)	(1,440)	(4,228)
Income tax expense	17	(1,094)	(393)	(1,416)
Net income	220	(2,129)	(1,047)	(2,812)
Total loans, leases, and loans held for sale, net of unearned income	127,712	192,804	127,712	192,804
Total assets	173,415	215,862	173,415	215,862
Total deposits	11,076	496	11,076	496
Average assets	148,210	226,326	166,872	256,343
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/(reversal of) loan losses	0	0	0	0
Mortgage income	0	0	0	0
Title revenue	6,846	6,190	11,873	10,931
Other non-interest income/(loss)	(3)	0	(3)	(6)
Allocated expenses	1,005	832	1,318	1,350
Non-interest expense	4,979	4,393	9,543	8,593
Income before income tax expense	859	965	1,010	983
Income tax expense	229	382	(349)	394
Net income	630	583	1,359	589
Total loans, leases, and loans held for sale, net of unearned income	0	0	0	0
Total assets	24,387	25,308	24,387	25,308
Total deposits	0	0	0	0
Average assets	$ 23,174	$ 24,185	$ 22,535	$ 24,032