Business Segments (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) Segment	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information
Number of business segments | Segment			3
Segment Reporting Information, Revenue for Reportable Segment [Abstract]
Interest and dividend income	$ 335,967	$ 303,823	$ 662,051	$ 574,366
Interest expense	80,628	47,710	156,228	85,364
Net interest income	255,339	256,113	505,823	489,002
Other Income and Expenses [Abstract]
Provision for/(reversal of) loan losses	10,755	7,696	24,518	15,907
Other non-interest income (expense)	33,486	33,373	68,921	63,317
Allocated expenses (income)	0	0	0	0
Non-interest expense	169,618	196,776	328,371	384,847
Income before income tax expense	133,791	105,581	264,268	186,754
Income tax expense	32,193	30,457	62,539	48,009
Net income	101,598	75,124	201,729	138,745
Total loans, leases, and loans held for sale, net of unearned income	23,543,298	22,154,626	23,543,298	22,154,626
Total assets	31,446,532	30,126,162	31,446,532	30,126,162	$ 30,833,015
Deposits	24,295,331	23,430,458	24,295,331	23,430,458	$ 23,763,431
Average assets	31,271,822	29,770,852	31,053,872	28,956,062
Title revenue
Other Income and Expenses [Abstract]
Non-interest income	6,895	6,846	12,120	11,873
Mortgage income
Other Income and Expenses [Abstract]
Non-interest income	18,444	13,721	30,293	23,316
Operating Segments | IBERIABANK
Segment Reporting Information, Revenue for Reportable Segment [Abstract]
Interest and dividend income	334,055	302,387	658,699	571,162
Interest expense	80,628	47,710	156,228	85,364
Net interest income	253,427	254,677	502,471	485,798
Other Income and Expenses [Abstract]
Provision for/(reversal of) loan losses	10,742	7,715	24,565	15,931
Other non-interest income (expense)	33,496	33,481	68,959	63,387
Allocated expenses (income)	(4,414)	(3,873)	(6,447)	(5,352)
Non-interest expense	151,139	179,831	294,894	351,422
Income before income tax expense	129,456	104,485	258,418	187,184
Income tax expense	31,134	30,211	61,109	48,751
Net income	98,322	74,274	197,309	138,433
Total loans, leases, and loans held for sale, net of unearned income	23,330,738	22,026,914	23,330,738	22,026,914
Total assets	31,165,205	29,928,360	31,165,205	29,928,360
Deposits	24,279,416	23,419,382	24,279,416	23,419,382
Average assets	31,024,832	29,599,468	30,838,856	28,766,655
Operating Segments | IMC
Segment Reporting Information, Revenue for Reportable Segment [Abstract]
Interest and dividend income	1,912	1,436	3,351	3,203
Interest expense	0	0	0	0
Net interest income	1,912	1,436	3,351	3,203
Other Income and Expenses [Abstract]
Provision for/(reversal of) loan losses	13	(19)	(47)	(24)
Other non-interest income (expense)	(10)	(105)	(22)	(67)
Allocated expenses (income)	3,250	2,868	4,750	4,034
Non-interest expense	13,590	11,966	24,131	23,882
Income before income tax expense	3,493	237	4,788	(1,440)
Income tax expense	836	17	1,143	(393)
Net income	2,657	220	3,645	(1,047)
Total loans, leases, and loans held for sale, net of unearned income	212,560	127,712	212,560	127,712
Total assets	255,106	173,415	255,106	173,415
Deposits	15,915	11,076	15,915	11,076
Average assets	221,891	148,210	190,205	166,872
Operating Segments | LTC
Segment Reporting Information, Revenue for Reportable Segment [Abstract]
Interest and dividend income	0	0	1	1
Interest expense	0	0	0	0
Net interest income	0	0	1	1
Other Income and Expenses [Abstract]
Provision for/(reversal of) loan losses	0	0	0	0
Other non-interest income (expense)	0	(3)	(16)	(3)
Allocated expenses (income)	1,164	1,005	1,697	1,318
Non-interest expense	4,889	4,979	9,346	9,543
Income before income tax expense	842	859	1,062	1,010
Income tax expense	223	229	287	(349)
Net income	619	630	775	1,359
Total loans, leases, and loans held for sale, net of unearned income	0	0	0	0
Total assets	26,221	24,387	26,221	24,387
Deposits	0	0	0	0
Average assets	25,099	23,174	24,811	22,535
Operating Segments | Title revenue | IBERIABANK
Other Income and Expenses [Abstract]
Non-interest income	0	0	0	0
Operating Segments | Title revenue | IMC
Other Income and Expenses [Abstract]
Non-interest income	0	0	0	0
Operating Segments | Title revenue | LTC
Other Income and Expenses [Abstract]
Non-interest income	6,895	6,846	12,120	11,873
Operating Segments | Mortgage income | IBERIABANK
Other Income and Expenses [Abstract]
Non-interest income	0	0	0	0
Operating Segments | Mortgage income | IMC
Other Income and Expenses [Abstract]
Non-interest income	18,444	13,721	30,293	23,316
Operating Segments | Mortgage income | LTC
Other Income and Expenses [Abstract]
Non-interest income	$ 0	$ 0	$ 0	$ 0