Goodwill and Other Intangible Assets - Schedule of Carrying Amount of Goodwill (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Goodwill And Other Intangibles [Line Items]
Balance, December 31, 2013	$ 517,526	$ 401,872
Goodwill acquired during the period	104,197	115,654
Balance, December 31, 2014	621,723	517,526
IBERIABANK [Member]
Goodwill And Other Intangibles [Line Items]
Balance, December 31, 2013	489,183	373,905
Goodwill acquired during the period	104,197	115,278
Balance, December 31, 2014	593,380	489,183
IMC [Member]
Goodwill And Other Intangibles [Line Items]
Balance, December 31, 2013			23,178
Balance, December 31, 2014	23,178	23,178	23,178
LTC [Member]
Goodwill And Other Intangibles [Line Items]
Balance, December 31, 2013		4,789
Goodwill acquired during the period		376
Balance, December 31, 2014	$ 5,165	$ 5,165