Business Segments - Schedule of Segment Reporting Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information
Interest and dividend income	$ 1,310,008	$ 1,221,629	$ 913,783
Interest expense	320,362	208,381	104,937
Net interest income	989,646	1,013,248	808,846
Provision for credit losses	41,657	40,385	51,708
Other non-interest income (expense)	145,402	81,989	116,605
Allocated expenses (income)	0	0	0
Non-interest expense	682,756	722,898	666,406
Income before income tax expense	499,593	402,527	292,879
Income tax expense	115,438	32,278	150,466
Net income	384,155	370,249	142,413
Total loans, leases, and loans held for sale, net of unearned income	24,234,856	22,627,549	20,213,097
Total assets	31,713,450	30,833,015	27,904,129
Total deposits	25,219,349	23,763,431	21,466,717
Average assets	31,377,868	29,578,026	24,480,656
Operating Segments | IBERIABANK
Segment Reporting Information
Interest and dividend income	1,301,901	1,215,668	906,521
Interest expense	320,362	208,381	104,937
Net interest income	981,539	1,007,287	801,584
Provision for credit losses	41,710	40,429	51,797
Other non-interest income (expense)	145,317	81,588	116,659
Allocated expenses (income)	(14,495)	(13,437)	(13,293)
Non-interest expense	611,794	660,804	575,865
Income before income tax expense	487,847	401,079	303,874
Income tax expense	112,624	32,436	156,407
Net income	375,223	368,643	147,467
Total loans, leases, and loans held for sale, net of unearned income	24,003,854	22,493,809	20,028,840
Total assets	31,413,998	30,645,000	27,672,906
Total deposits	25,200,205	23,754,512	21,462,776
Average assets	31,121,870	29,400,755	24,228,436
Operating Segments | Mortgage
Segment Reporting Information
Interest and dividend income	8,104	5,958	7,260
Interest expense	0	0	0
Net interest income	8,104	5,958	7,260
Provision for credit losses	(53)	(44)	(89)
Other non-interest income (expense)	104	(95)	(42)
Allocated expenses (income)	10,640	9,847	10,041
Non-interest expense	51,734	43,021	73,587
Income before income tax expense	8,917	(537)	(12,751)
Income tax expense	2,051	(52)	(5,771)
Net income	6,866	(485)	(6,980)
Total loans, leases, and loans held for sale, net of unearned income	231,002	133,740	184,257
Total assets	271,382	162,599	208,710
Total deposits	19,144	8,919	3,941
Average assets	230,241	153,717	229,364
Operating Segments | LTC
Segment Reporting Information
Interest and dividend income	3	3	2
Interest expense	0	0	0
Net interest income	3	3	2
Provision for credit losses	0	0	0
Other non-interest income (expense)	(19)	496	(12)
Allocated expenses (income)	3,855	3,590	3,252
Non-interest expense	19,228	19,073	16,954
Income before income tax expense	2,829	1,985	1,756
Income tax expense	763	(106)	(170)
Net income	2,066	2,091	1,926
Total loans, leases, and loans held for sale, net of unearned income	0	0	0
Total assets	28,070	25,416	22,513
Total deposits	0	0	0
Average assets	25,757	23,554	22,856
Mortgage income
Segment Reporting Information
Non-interest income	63,030	46,424	63,570
Mortgage income | Operating Segments | IBERIABANK
Segment Reporting Information
Non-interest income	0	0	0
Mortgage income | Operating Segments | Mortgage
Segment Reporting Information
Non-interest income	63,030	46,424	63,570
Mortgage income | Operating Segments | LTC
Segment Reporting Information
Non-interest income	0	0	0
Title revenue
Segment Reporting Information
Non-interest income	25,928	24,149	21,972
Title revenue | Operating Segments | IBERIABANK
Segment Reporting Information
Non-interest income	0	0	0
Title revenue | Operating Segments | Mortgage
Segment Reporting Information
Non-interest income	0	0	0
Title revenue | Operating Segments | LTC
Segment Reporting Information
Non-interest income	$ 25,928	$ 24,149	$ 21,972