Goodwill and Other Intangible Assets - Schedule of Carrying Amount of Goodwill (Detail) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Goodwill
Balance at beginning of the period				$ 1,188,902		$ 726,856
Goodwill acquired during the year (preliminary allocation)						462,046
Goodwill acquired during the year (preliminary allocation) and adjustments				34,870
Balance at end of the period	$ 1,223,772	$ 1,223,772		1,223,772	$ 1,223,772	1,188,902
Operating Segments | IBERIABANK
Goodwill
Balance at beginning of the period				1,160,559		698,513
Goodwill acquired during the year (preliminary allocation)						462,046
Goodwill acquired during the year (preliminary allocation) and adjustments				31,490
Balance at end of the period	1,192,049	1,192,049		1,192,049	1,192,049	1,160,559
Operating Segments | IMC
Goodwill
Balance at beginning of the period				23,178		23,178
Goodwill acquired during the year (preliminary allocation)						0
Goodwill acquired during the year (preliminary allocation) and adjustments				0
Balance at end of the period	23,178	23,178		23,178	23,178	23,178
Operating Segments | LTC
Goodwill
Balance at beginning of the period				5,165		5,165
Goodwill acquired during the year (preliminary allocation)						0
Goodwill acquired during the year (preliminary allocation) and adjustments				3,380
Balance at end of the period	8,545	8,545		8,545	8,545	$ 5,165
Gibraltar Private Bank & Trust Company
Goodwill
Goodwill acquired during the year (preliminary allocation) and adjustments	10,000	52,500
Balance at end of the period	52,500	52,500		52,500	52,500
Solomon Parks Title & Escrow
Goodwill
Goodwill acquired during the year (preliminary allocation) and adjustments			$ 3,400		3,400
Sabadell United
Goodwill
Goodwill acquired during the year (preliminary allocation) and adjustments	22,400			21,000
Balance at end of the period	$ 441,000	$ 441,000		$ 441,000	$ 441,000