Goodwill and Other Intangible Assets - Schedule of Carrying Amount of Goodwill (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Goodwill
Balance at beginning of the period	$ 726,856	$ 724,603
Goodwill adjustments during the year		2,253
Goodwill acquired during the year (preliminary allocation)	431,827
Balance at end of the period	1,158,683	726,856
Operating Segments | IBERIABANK
Goodwill
Balance at beginning of the period	698,513	696,260
Goodwill adjustments during the year		2,253
Goodwill acquired during the year (preliminary allocation)	431,827
Balance at end of the period	1,130,340	698,513
Operating Segments | IMC
Goodwill
Balance at beginning of the period	23,178	23,178
Goodwill adjustments during the year		0
Goodwill acquired during the year (preliminary allocation)	0
Balance at end of the period	23,178	23,178
Operating Segments | LTC
Goodwill
Balance at beginning of the period	5,165	5,165
Goodwill adjustments during the year		0
Goodwill acquired during the year (preliminary allocation)	0
Balance at end of the period	$ 5,165	$ 5,165