Goodwill and Other Intangible Assets - Schedule of Carrying Amount of Goodwill (Detail) $ in Thousands	Mar. 31, 2020 USD ($)
Goodwill [Roll Forward]
Balance at beginning of the period	$ 1,235,533
Balance at end of the period	1,235,533
Operating Segments
Goodwill [Roll Forward]
Balance at beginning of the period	1,235,533
Balance at end of the period	1,235,533
Operating Segments | IBERIABANK
Goodwill [Roll Forward]
Balance at beginning of the period	1,203,810
Balance at end of the period	1,203,810
Operating Segments | IMC
Goodwill [Roll Forward]
Balance at beginning of the period	23,178
Balance at end of the period	23,178
Operating Segments | LTC
Goodwill [Roll Forward]
Balance at beginning of the period	8,545
Balance at end of the period	$ 8,545