Goodwill and Other Intangible Assets - Schedule of Carrying Amount of Goodwill (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Jun. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Goodwill
Balance at beginning of the period			$ 1,188,902	$ 1,188,902	$ 726,856
Goodwill acquired during the year					462,046
Goodwill acquired during the year (preliminary allocation) and adjustments				32,001
Balance at end of the period	$ 1,220,903			1,220,903	1,188,902
Operating Segments | IBERIABANK
Goodwill
Balance at beginning of the period			1,160,559	1,160,559	698,513
Goodwill acquired during the year					462,046
Goodwill acquired during the year (preliminary allocation) and adjustments				28,621
Balance at end of the period	1,189,180			1,189,180	1,160,559
Operating Segments | IMC
Goodwill
Balance at beginning of the period			23,178	23,178	23,178
Goodwill acquired during the year					0
Goodwill acquired during the year (preliminary allocation) and adjustments				0
Balance at end of the period	23,178			23,178	23,178
Operating Segments | LTC
Goodwill
Balance at beginning of the period			5,165	5,165	5,165
Goodwill acquired during the year					0
Goodwill acquired during the year (preliminary allocation) and adjustments				3,380
Balance at end of the period	8,545			8,545	$ 5,165
Gibraltar Private Bank & Trust Company
Goodwill
Goodwill acquired during the year (preliminary allocation) and adjustments	2,900
Balance at end of the period	49,600			49,600
Solomon Parks Title & Escrow
Goodwill
Goodwill acquired during the year (preliminary allocation) and adjustments		$ 3,400		$ 3,400
Sabadell United
Goodwill
Balance at beginning of the period	$ 441,000
Goodwill acquired during the year (preliminary allocation) and adjustments			21,000
Balance at end of the period			$ 441,000