Shareholders' Equity, Capital Ratios and Other Regulatory Matters - Actual Capital Amounts and Ratios (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Consolidated
Tier 1 Leverage
Minimum amount	$ 784,103	$ 751,798
Minimum ratio	4.00%	4.00%
Actual amount	$ 1,901,866	$ 1,790,034
Actual ratio	9.70%	9.52%
Common Equity Tier 1 (CET1)
Minimum amount	$ 785,140	$ 752,610
Minimum ratio	4.50%	4.50%
Actual amount	$ 1,769,769	$ 1,684,097
Actual ratio	10.14%	10.07%
Tier 1 Risk-Based Capital
Minimum amount	$ 1,046,854	$ 1,003,479
Minimum ratio	6.00%	6.00%
Actual amount	$ 1,901,866	$ 1,790,034
Actual ratio	10.90%	10.70%
Total Risk-Based Capital
Minimum amount	$ 1,395,805	$ 1,337,973
Minimum ratio	8.00%	8.00%
Actual amount	$ 2,178,549	$ 2,029,932
Actual ratio	12.49%	12.14%
IBERIABANK
Tier 1 Leverage
Minimum amount	$ 781,582	$ 749,226
Minimum ratio	4.00%	4.00%
Well capitalized amount	$ 976,978	$ 936,532
Well capitalized ratio	5.00%	5.00%
Actual amount	$ 1,831,469	$ 1,691,022
Actual ratio	9.37%	9.03%
Common Equity Tier 1 (CET1)
Minimum amount	$ 783,333	$ 750,660
Minimum ratio	4.50%	4.50%
Well capitalized amount	$ 1,131,481	$ 1,084,287
Well capitalized ratio	6.50%	6.50%
Actual amount	$ 1,831,469	$ 1,691,022
Actual ratio	10.52%	10.14%
Tier 1 Risk-Based Capital
Minimum amount	$ 1,044,444	$ 1,000,880
Minimum ratio	6.00%	6.00%
Well capitalized amount	$ 1,392,591	$ 1,334,507
Well capitalized ratio	8.00%	8.00%
Actual amount	$ 1,831,469	$ 1,691,022
Actual ratio	10.52%	10.14%
Total Risk-Based Capital
Minimum amount	$ 1,392,591	$ 1,334,507
Minimum ratio	8.00%	8.00%
Well capitalized amount	$ 1,740,739	$ 1,668,133
Well capitalized ratio	10.00%	10.00%
Actual amount	$ 1,991,651	$ 1,843,545
Actual ratio	11.44%	11.05%