Shareholders' Equity, Capital Ratios and Other Regulatory Matters - Actual Capital Amounts and Ratios (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Total Risk-Based Capital
Capital Conversion Buffer	4.34%
Consolidated
Tier 1 Leverage
Minimum amount	$ 1,208,894	$ 1,168,343
Minimum ratio	4.00%	4.00%
Actual amount	$ 2,955,775	$ 2,812,863
Actual ratio	9.78%	9.63%
Common Equity Tier 1 (CET1)
Minimum amount	$ 1,179,356	$ 1,125,405
Minimum ratio	4.50%	4.50%
Actual amount	$ 2,727,290	$ 2,680,766
Actual Ratio	10.41%	10.72%
Tier 1 Risk-Based Capital
Minimum amount	$ 1,572,475	$ 1,500,540
Minimum ratio	6.00%	6.00%
Actual amount	$ 2,955,775	$ 2,812,863
Actual ratio	11.28%	11.25%
Total Risk-Based Capital
Minimum amount	$ 2,096,633	$ 2,000,720
Minimum ratio	8.00%	8.00%
Actual amount	$ 3,234,654	$ 3,084,764
Actual ratio	12.34%	12.33%
IBERIABANK
Tier 1 Leverage
Minimum amount	$ 1,206,257	$ 1,165,537
Minimum ratio	4.00%	4.00%
Well capitalized amount	$ 1,507,821	$ 1,456,921
Well capitalized ratio	5.00%	5.00%
Actual amount	$ 2,861,883	$ 2,733,099
Actual ratio	9.49%	9.38%
Common Equity Tier 1 (CET1)
Minimum amount	$ 1,175,984	$ 1,122,712
Minimum ratio	4.50%	4.50%
Well capitalized amount	$ 1,698,643	$ 1,621,695
Well capitalized ratio	6.50%	6.50%
Actual amount	$ 2,861,883	$ 2,733,099
Actual Ratio	10.95%	10.95%
Tier 1 Risk-Based Capital
Minimum amount	$ 1,567,978	$ 1,496,949
Minimum ratio	6.00%	6.00%
Well capitalized amount	$ 2,090,638	$ 1,995,932
Well capitalized ratio	8.00%	8.00%
Actual amount	$ 2,861,883	$ 2,733,099
Actual ratio	10.95%	10.95%
Total Risk-Based Capital
Minimum amount	$ 2,090,638	$ 1,995,932
Minimum ratio	8.00%	8.00%
Well capitalized amount	$ 2,613,297	$ 2,494,915
Well capitalized ratio	10.00%	10.00%
Actual amount	$ 3,024,262	$ 2,888,500
Actual ratio	11.57%	11.58%
Capital Conversion Buffer	3.57%