Shareholders' Equity, Capital Ratios and Other Regulatory Matters - Actual Capital Amounts and Ratios (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Consolidated
Tier 1 Leverage
Minimum amount	$ 995,849	$ 818,440
Minimum ratio	4.00%	4.00%
Actual amount	$ 2,532,164	$ 2,221,528
Actual ratio	10.17%	10.86%
Common Equity Tier 1 (CET1)
Minimum amount	$ 988,546	$ 794,334
Minimum ratio	4.50%	4.50%
Actual amount	$ 2,400,067	$ 2,089,431
Actual ratio	10.93%	11.84%
Tier 1 Risk-Based Capital (1)
Minimum amount	$ 1,318,062	$ 1,059,112
Minimum ratio	6.00%	6.00%
Actual amount	$ 2,532,164	$ 2,221,528
Actual ratio	11.53%	12.59%
Total Risk-Based Capital (1)
Minimum amount	$ 1,757,416	$ 1,412,149
Minimum ratio	8.00%	8.00%
Actual amount	$ 2,806,324	$ 2,493,988
Actual ratio	12.78%	14.13%
IBERIABANK
Tier 1 Leverage
Minimum amount	$ 993,353	$ 816,152
Minimum ratio	4.00%	4.00%
Well capitalized amount	$ 1,241,691	$ 1,020,190
Well capitalized ratio	5.00%	5.00%
Actual amount	$ 2,435,190	$ 1,878,703
Actual ratio	9.81%	9.21%
Common Equity Tier 1 (CET1)
Minimum amount	$ 986,055	$ 792,111
Minimum ratio	4.50%	4.50%
Well capitalized amount	$ 1,424,301	$ 1,144,160
Well capitalized ratio	6.50%	6.50%
Actual amount	$ 2,435,190	$ 1,878,703
Actual ratio	11.11%	10.67%
Tier 1 Risk-Based Capital (1)
Minimum amount	$ 1,314,739	$ 1,056,147
Minimum ratio	6.00%	6.00%
Well capitalized amount	$ 1,752,986	$ 1,408,197
Well capitalized ratio	8.00%	8.00%
Actual amount	$ 2,435,190	$ 1,878,703
Actual ratio	11.11%	10.67%
Total Risk-Based Capital (1)
Minimum amount	$ 1,752,986	$ 1,408,197
Minimum ratio	8.00%	8.00%
Well capitalized amount	$ 2,191,232	$ 1,760,246
Well capitalized ratio	10.00%	10.00%
Actual amount	$ 2,592,850	$ 2,034,663
Actual ratio	11.83%	11.56%