Shareholders' Equity, Capital Ratios and Other Regulatory Matters - Actual Capital Amounts and Ratios (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Total Risk-Based Capital
Capital Conversion Buffer	4.33%
Consolidated
Tier 1 Leverage
Minimum amount	$ 1,184,075	$ 1,168,343
Minimum ratio	4.00%	4.00%
Actual amount	$ 2,863,388	$ 2,812,863
Actual ratio	9.67%	9.63%
Common Equity Tier 1 (CET1)
Minimum amount	$ 1,145,643	$ 1,125,405
Minimum ratio	4.50%	4.50%
Actual amount	$ 2,731,291	$ 2,680,766
Actual Ratio	10.73%	10.72%
Tier 1 Risk-Based Capital
Minimum amount	$ 1,527,524	$ 1,500,540
Minimum ratio	6.00%	6.00%
Actual amount	$ 2,863,388	$ 2,812,863
Actual ratio	11.25%	11.25%
Total Risk-Based Capital
Minimum amount	$ 2,036,699	$ 2,000,720
Minimum ratio	8.00%	8.00%
Actual amount	$ 3,138,835	$ 3,084,764
Actual ratio	12.33%	12.33%
IBERIABANK
Tier 1 Leverage
Minimum amount	$ 1,181,108	$ 1,165,537
Minimum ratio	4.00%	4.00%
Well capitalized amount	$ 1,476,386	$ 1,456,921
Well capitalized ratio	5.00%	5.00%
Actual amount	$ 2,795,498	$ 2,733,099
Actual ratio	9.47%	9.38%
Common Equity Tier 1 (CET1)
Minimum amount	$ 1,143,315	$ 1,122,712
Minimum ratio	4.50%	4.50%
Well capitalized amount	$ 1,651,455	$ 1,621,695
Well capitalized ratio	6.50%	6.50%
Actual amount	$ 2,795,498	$ 2,733,099
Actual Ratio	11.00%	10.95%
Tier 1 Risk-Based Capital
Minimum amount	$ 1,524,420	$ 1,496,949
Minimum ratio	6.00%	6.00%
Well capitalized amount	$ 2,032,560	$ 1,995,932
Well capitalized ratio	8.00%	8.00%
Actual amount	$ 2,795,498	$ 2,733,099
Actual ratio	11.00%	10.95%
Total Risk-Based Capital
Minimum amount	$ 2,032,560	$ 1,995,932
Minimum ratio	8.00%	8.00%
Well capitalized amount	$ 2,540,700	$ 2,494,915
Well capitalized ratio	10.00%	10.00%
Actual amount	$ 2,954,445	$ 2,888,500
Actual ratio	11.63%	11.58%
Capital Conversion Buffer	3.63%