Business Segments - Schedule of Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Segment Reporting Information
Interest and dividend income	$ 246,972	$ 180,504	$ 644,080	$ 536,134
Interest expense	30,089	17,087	70,736	48,561
Net interest income	216,883	163,417	573,344	487,573
Provision for loan losses	18,514	12,484	36,718	39,255
Mortgage income	16,050	21,807	49,895	67,738
Service charges on deposit accounts	12,534	11,066	35,097	32,957
Title revenue	5,643	6,001	16,574	16,881
Other non-interest income	18,840	20,947	54,813	63,007
Allocated expenses	0	0	0	0
Non-interest expense	202,986	138,139	491,512	415,095
Income before income tax expense	48,450	72,615	201,493	213,806
Income tax expense	18,806	24,547	69,358	72,159
Net income	29,644	48,068	132,135	141,647
Total loans and loans held for sale, net of unearned income	19,936,303	15,135,365	19,936,303	15,135,365
Total assets	27,976,635	20,788,566	27,976,635	20,788,566	$ 21,659,190
Total deposits	21,334,271	16,522,517	21,334,271	16,522,517	$ 17,408,283
Average assets	26,096,921	20,392,828	23,282,834	20,020,715
Operating Segments | IBERIABANK
Segment Reporting Information
Interest and dividend income	245,166	178,521	638,683	530,409
Interest expense	30,089	15,944	70,736	45,380
Net interest income	215,077	162,577	567,947	485,029
Provision for loan losses	18,524	12,443	36,816	39,214
Mortgage income	0	0	0	0
Service charges on deposit accounts	12,534	11,066	35,097	32,957
Title revenue	0	0	0	0
Other non-interest income	18,853	20,946	54,853	62,999
Allocated expenses	(3,881)	(3,914)	(9,377)	(10,468)
Non-interest expense	176,530	127,690	418,879	367,985
Income before income tax expense	55,291	58,370	211,579	184,254
Income tax expense	21,076	19,282	72,650	60,841
Net income	34,215	39,088	138,929	123,413
Total loans and loans held for sale, net of unearned income	19,743,749	14,957,337	19,743,749	14,957,337
Total assets	27,738,588	20,537,304	27,738,588	20,537,304
Total deposits	21,333,190	16,513,867	21,333,190	16,513,867
Average assets	25,869,559	20,061,071	23,020,324	19,705,052
Operating Segments | IMC
Segment Reporting Information
Interest and dividend income	1,805	1,982	5,395	5,723
Interest expense	0	1,143	0	3,181
Net interest income	1,805	839	5,395	2,542
Provision for loan losses	(10)	41	(98)	41
Mortgage income	16,050	21,807	49,895	67,738
Service charges on deposit accounts	0	0	0	0
Title revenue	0	0	0	0
Other non-interest income	(9)	1	(30)	8
Allocated expenses	2,911	3,010	7,057	8,007
Non-interest expense	22,170	5,891	59,754	33,909
Income before income tax expense	(7,225)	13,705	(11,453)	28,331
Income tax expense	(2,424)	5,048	(3,840)	10,826
Net income	(4,801)	8,657	(7,613)	17,505
Total loans and loans held for sale, net of unearned income	192,554	178,028	192,554	178,028
Total assets	217,056	226,906	217,056	226,906
Total deposits	1,081	8,650	1,081	8,650
Average assets	204,920	305,996	239,014	289,037
Operating Segments | LTC
Segment Reporting Information
Interest and dividend income	1	1	2	2
Interest expense	0	0	0	0
Net interest income	1	1	2	2
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	5,643	6,001	16,574	16,881
Other non-interest income	(4)	0	(10)	0
Allocated expenses	970	904	2,320	2,461
Non-interest expense	4,286	4,558	12,879	13,201
Income before income tax expense	384	540	1,367	1,221
Income tax expense	154	217	548	492
Net income	230	323	819	729
Total loans and loans held for sale, net of unearned income	0	0	0	0
Total assets	20,991	24,356	20,991	24,356
Total deposits	0	0	0	0
Average assets	$ 22,442	$ 25,761	$ 23,496	$ 26,626