Shareholders' Equity, Capital Ratios and Other Regulatory Matters - Actual Capital Amounts and Ratios (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Consolidated
Tier 1 Leverage
Minimum amount	$ 1,076,933	$ 1,073,381
Minimum ratio	4.00%	4.00%
Actual amount	$ 2,685,315	$ 2,509,496
Actual ratio	9.97%	9.35%
Common Equity Tier 1 (CET1)
Minimum amount	$ 1,067,284	$ 1,011,732
Minimum ratio	4.50%	4.50%
Actual amount	$ 2,553,218	$ 2,377,398
Actual ratio	10.77%	10.57%
Tier 1 Risk-Based Capital (1)
Minimum amount	$ 1,423,046	$ 1,348,977
Minimum ratio	6.00%	6.00%
Actual amount	$ 2,685,315	$ 2,509,496
Actual ratio	11.32%	11.16%
Total Risk-Based Capital (1)
Minimum amount	$ 1,897,394	$ 1,798,635
Minimum ratio	8.00%	8.00%
Actual amount	$ 2,959,774	$ 2,780,095
Actual ratio	12.48%	12.37%
IBERIABANK
Tier 1 Leverage
Minimum amount	$ 1,074,579	$ 1,070,789
Minimum ratio	4.00%	4.00%
Well capitalized amount	$ 1,343,223	$ 1,338,487
Well capitalized ratio	5.00%	5.00%
Actual amount	$ 2,640,916	$ 2,437,275
Actual ratio	9.83%	9.10%
Common Equity Tier 1 (CET1)
Minimum amount	$ 1,065,059	$ 1,009,553
Minimum ratio	4.50%	4.50%
Well capitalized amount	$ 1,538,418	$ 1,458,243
Well capitalized ratio	6.50%	6.50%
Actual amount	$ 2,640,916	$ 2,437,275
Actual ratio	11.16%	10.86%
Tier 1 Risk-Based Capital (1)
Minimum amount	$ 1,420,078	$ 1,346,070
Minimum ratio	6.00%	6.00%
Well capitalized amount	$ 1,893,437	$ 1,794,760
Well capitalized ratio	8.00%	8.00%
Actual amount	$ 2,640,916	$ 2,437,275
Actual ratio	11.16%	10.86%
Total Risk-Based Capital (1)
Minimum amount	$ 1,893,437	$ 1,794,760
Minimum ratio	8.00%	8.00%
Well capitalized amount	$ 2,366,797	$ 2,243,450
Well capitalized ratio	10.00%	10.00%
Actual amount	$ 2,798,875	$ 2,591,374
Actual ratio	11.83%	11.55%