Shareholders' Equity, Capital Ratios and Other Regulatory Matters - Actual Capital Amounts and Ratios (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Consolidated
Tier 1 Leverage
Minimum amount	$ 1,220,289	$ 1,168,343
Minimum ratio	4.00%	4.00%
Actual amount	$ 3,021,004	$ 2,812,863
Actual ratio	9.90%	9.63%
Common Equity Tier 1 (CET1)
Minimum amount	$ 1,194,765	$ 1,125,405
Minimum ratio	4.50%	4.50%
Actual amount	$ 2,792,520	$ 2,680,766
Actual ratio	10.52%	10.72%
Tier 1 Risk-Based Capital
Minimum amount	$ 1,593,020	$ 1,500,540
Minimum ratio	6.00%	6.00%
Actual amount	$ 3,021,004	$ 2,812,863
Actual ratio	11.38%	11.25%
Total Risk-Based Capital
Minimum amount	$ 2,124,026	$ 2,000,720
Minimum ratio	8.00%	8.00%
Actual amount	$ 3,300,730	$ 3,084,764
Actual ratio	12.43%	12.33%
IBERIABANK
Tier 1 Leverage
Minimum amount	$ 1,217,638	$ 1,165,537
Minimum ratio	4.00%	4.00%
Well capitalized amount	$ 1,522,048	$ 1,456,921
Well capitalized ratio	5.00%	5.00%
Actual amount	$ 2,949,533	$ 2,733,099
Actual ratio	9.69%	9.38%
Common Equity Tier 1 (CET1)
Minimum amount	$ 1,191,062	$ 1,122,712
Minimum ratio	4.50%	4.50%
Well capitalized amount	$ 1,720,423	$ 1,621,695
Well capitalized ratio	6.50%	6.50%
Actual amount	$ 2,949,533	$ 2,733,099
Actual ratio	11.14%	10.95%
Tier 1 Risk-Based Capital
Minimum amount	$ 1,588,083	$ 1,496,949
Minimum ratio	6.00%	6.00%
Well capitalized amount	$ 2,117,444	$ 1,995,932
Well capitalized ratio	8.00%	8.00%
Actual amount	$ 2,949,533	$ 2,733,099
Actual ratio	11.14%	10.95%
Total Risk-Based Capital
Minimum amount	$ 2,117,444	$ 1,995,932
Minimum ratio	8.00%	8.00%
Well capitalized amount	$ 2,648,805	$ 2,494,915
Well capitalized ratio	10.00%	10.00%
Actual amount	$ 3,112,758	$ 2,888,500
Actual ratio	11.76%	11.58%