Shareholders' Equity, Capital Ratios and Other Regulatory Matters - Actual Capital Amounts and Ratios (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Consolidated
Tier 1 Leverage
Minimum amount	$ 1,156,436	$ 1,073,381
Minimum ratio	4.00%	4.00%
Actual amount	$ 2,789,740	$ 2,509,496
Actual ratio	9.65%	9.35%
Common Equity Tier 1 (CET1)
Minimum amount	$ 1,107,934	$ 1,011,732
Minimum ratio	4.50%	4.50%
Actual amount	$ 2,657,643	$ 2,377,398
Actual ratio	10.79%	10.57%
Tier 1 Risk-Based Capital (1)
Minimum amount	$ 1,477,245	$ 1,348,977
Minimum ratio	6.00%	6.00%
Actual amount	$ 2,789,740	$ 2,509,496
Actual ratio	11.33%	11.16%
Total Risk-Based Capital (1)
Minimum amount	$ 1,969,660	$ 1,798,635
Minimum ratio	8.00%	8.00%
Actual amount	$ 3,057,911	$ 2,780,095
Actual ratio	12.42%	12.37%
IBERIABANK
Tier 1 Leverage
Minimum amount	$ 1,153,671	$ 1,070,789
Minimum ratio	4.00%	4.00%
Well capitalized amount	$ 1,442,089	$ 1,338,487
Well capitalized ratio	5.00%	5.00%
Actual amount	$ 2,768,917	$ 2,437,275
Actual ratio	9.60%	9.10%
Common Equity Tier 1 (CET1)
Minimum amount	$ 1,105,391	$ 1,009,553
Minimum ratio	4.50%	4.50%
Well capitalized amount	$ 1,596,676	$ 1,458,243
Well capitalized ratio	6.50%	6.50%
Actual amount	$ 2,768,917	$ 2,437,275
Actual ratio	11.27%	10.86%
Tier 1 Risk-Based Capital (1)
Minimum amount	$ 1,473,855	$ 1,346,070
Minimum ratio	6.00%	6.00%
Well capitalized amount	$ 1,965,140	$ 1,794,760
Well capitalized ratio	8.00%	8.00%
Actual amount	$ 2,768,917	$ 2,437,275
Actual ratio	11.27%	10.86%
Total Risk-Based Capital (1)
Minimum amount	$ 1,965,140	$ 1,794,760
Minimum ratio	8.00%	8.00%
Well capitalized amount	$ 2,456,425	$ 2,243,450
Well capitalized ratio	10.00%	10.00%
Actual amount	$ 2,920,587	$ 2,591,374
Actual ratio	11.89%	11.55%