Shareholders' Equity, Capital Ratios and Other Regulatory Matters - Actual Capital Amounts and Ratios (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Total Risk-Based Capital
Capital Conversion Buffer	4.33%
Consolidated
Tier 1 Leverage
Minimum amount	$ 1,199,612	$ 1,168,343
Minimum ratio	4.00%	4.00%
Actual amount	$ 2,913,362	$ 2,812,863
Actual ratio	9.71%	9.63%
Common Equity Tier 1 (CET1)
Minimum amount	$ 1,164,678	$ 1,125,405
Minimum ratio	4.50%	4.50%
Actual amount	$ 2,684,877	$ 2,680,766
Actual Ratio	10.38%	10.72%
Tier 1 Risk-Based Capital
Minimum amount	$ 1,552,904	$ 1,500,540
Minimum ratio	6.00%	6.00%
Actual amount	$ 2,913,362	$ 2,812,863
Actual ratio	11.26%	11.25%
Total Risk-Based Capital
Minimum amount	$ 2,070,539	$ 2,000,720
Minimum ratio	8.00%	8.00%
Actual amount	$ 3,191,529	$ 3,084,764
Actual ratio	12.33%	12.33%
IBERIABANK
Tier 1 Leverage
Minimum amount	$ 1,196,979	$ 1,165,537
Minimum ratio	4.00%	4.00%
Well capitalized amount	$ 1,496,224	$ 1,456,921
Well capitalized ratio	5.00%	5.00%
Actual amount	$ 2,857,238	$ 2,733,099
Actual ratio	9.55%	9.38%
Common Equity Tier 1 (CET1)
Minimum amount	$ 1,161,409	$ 1,122,712
Minimum ratio	4.50%	4.50%
Well capitalized amount	$ 1,677,591	$ 1,621,695
Well capitalized ratio	6.50%	6.50%
Actual amount	$ 2,857,238	$ 2,733,099
Actual Ratio	11.07%	10.95%
Tier 1 Risk-Based Capital
Minimum amount	$ 1,548,546	$ 1,496,949
Minimum ratio	6.00%	6.00%
Well capitalized amount	$ 2,064,728	$ 1,995,932
Well capitalized ratio	8.00%	8.00%
Actual amount	$ 2,857,238	$ 2,733,099
Actual ratio	11.07%	10.95%
Total Risk-Based Capital
Minimum amount	$ 2,064,728	$ 1,995,932
Minimum ratio	8.00%	8.00%
Well capitalized amount	$ 2,580,910	$ 2,494,915
Well capitalized ratio	10.00%	10.00%
Actual amount	$ 3,018,905	$ 2,888,500
Actual ratio	11.70%	11.58%
Capital Conversion Buffer	3.70%