Goodwill and Other Intangible Assets - Schedule of Carrying Amount of Goodwill (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Goodwill [Roll Forward]
Beginning balance	$ 724,603	$ 517,526
Goodwill adjustments during the period	2,253	207,077
Ending balance	726,856	724,603
IBERIABANK
Goodwill [Roll Forward]
Beginning balance	696,260	489,183
Goodwill adjustments during the period	2,253	207,077
Ending balance	698,513	696,260
IMC
Goodwill [Roll Forward]
Beginning balance	23,178	23,178
Goodwill adjustments during the period	0	0
Ending balance	23,178	23,178
LTC
Goodwill [Roll Forward]
Beginning balance	5,165	5,165
Goodwill adjustments during the period	0	0
Ending balance	$ 5,165	$ 5,165