Goodwill and Other Intangible Assets - Schedule of Carrying Amount of Goodwill (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Goodwill [Roll Forward]
Beginning balance	$ 517,526	$ 401,872
Goodwill acquired during the period	201,937	115,654
Ending balance	719,463	517,526
Operating Segments | IBERIABANK
Goodwill [Roll Forward]
Beginning balance	489,183	373,905
Goodwill acquired during the period	201,937	115,278
Ending balance	691,120	489,183
Operating Segments | IMC
Goodwill [Roll Forward]
Beginning balance	23,178	23,178
Goodwill acquired during the period	0	0
Ending balance	23,178	23,178
Operating Segments | LTC
Goodwill [Roll Forward]
Beginning balance	5,165	4,789
Goodwill acquired during the period	0	376
Ending balance	$ 5,165	$ 5,165