Goodwill and Other Acquired Intangible Assets - Schedule of Carrying Amount of Goodwill (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Goodwill
Balance at beginning of the period	$ 726,856	$ 724,603
Goodwill adjustments during the year		2,253
Goodwill adjustments during the year	0
Balance at end of the period	726,856	726,856
Operating Segments | IBERIABANK
Goodwill
Balance at beginning of the period	698,513	696,260
Goodwill adjustments during the year		2,253
Goodwill adjustments during the year	0
Balance at end of the period	698,513	698,513
Operating Segments | IMC
Goodwill
Balance at beginning of the period	23,178	23,178
Goodwill adjustments during the year		0
Goodwill adjustments during the year	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 adjustments during the year	0
Balance at end of the period	$ 5,165	$ 5,165