Goodwill and Other Intangible Assets - Schedule of Carrying Amount of Goodwill (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Goodwill
Balance at beginning of the period	$ 1,235,533	$ 1,188,902
Goodwill acquired and adjustments during the year	0	46,631
Balance at end of the period	1,235,533	1,235,533
Operating Segments | IBERIABANK
Goodwill
Balance at beginning of the period	1,203,810	1,160,559
Goodwill acquired and adjustments during the year	0	43,251
Balance at end of the period	1,203,810	1,203,810
Operating Segments | IMC
Goodwill
Balance at beginning of the period	23,178	23,178
Goodwill acquired and adjustments during the year	0	0
Balance at end of the period	23,178	23,178
Operating Segments | LTC
Goodwill
Balance at beginning of the period	8,545	5,165
Goodwill acquired and adjustments during the year	0	3,380
Balance at end of the period	$ 8,545	$ 8,545