Shareholders' Equity, Capital Ratios and Other Regulatory Matters - Actual Capital Amounts and Ratios (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Total Risk-Based Capital
Capital Conversion Buffer	4.48%
Consolidated
Tier 1 Leverage
Minimum amount	$ 1,228,618	$ 1,220,289
Minimum ratio	4.00%	4.00%
Actual amount	$ 3,051,243	$ 3,021,004
Actual ratio	9.93%	9.90%
Common Equity Tier 1 (CET1)
Minimum amount	$ 1,216,954	$ 1,194,765
Minimum ratio	4.50%	4.50%
Actual amount	$ 2,822,758	$ 2,792,520
Actual Ratio	10.44%	10.52%
Tier 1 Risk-Based Capital
Minimum amount	$ 1,622,605	$ 1,593,020
Minimum ratio	6.00%	6.00%
Actual amount	$ 3,051,243	$ 3,021,004
Actual ratio	11.28%	11.38%
Total Risk-Based Capital
Minimum amount	$ 2,163,474	$ 2,124,026
Minimum ratio	8.00%	8.00%
Actual amount	$ 3,375,550	$ 3,300,730
Actual ratio	12.48%	12.43%
IBERIABANK
Tier 1 Leverage
Minimum amount	$ 1,225,516	$ 1,217,638
Minimum ratio	4.00%	4.00%
Well capitalized amount	$ 1,531,895	$ 1,522,048
Well capitalized ratio	5.00%	5.00%
Actual amount	$ 3,008,038	$ 2,949,533
Actual ratio	9.82%	9.69%
Common Equity Tier 1 (CET1)
Minimum amount	$ 1,212,501	$ 1,191,062
Minimum ratio	4.50%	4.50%
Well capitalized amount	$ 1,751,390	$ 1,720,423
Well capitalized ratio	6.50%	6.50%
Actual amount	$ 3,008,038	$ 2,949,533
Actual Ratio	11.16%	11.14%
Tier 1 Risk-Based Capital
Minimum amount	$ 1,616,668	$ 1,588,083
Minimum ratio	6.00%	6.00%
Well capitalized amount	$ 2,155,557	$ 2,117,444
Well capitalized ratio	8.00%	8.00%
Actual amount	$ 3,008,038	$ 2,949,533
Actual ratio	11.16%	11.14%
Total Risk-Based Capital
Minimum amount	$ 2,155,557	$ 2,117,444
Minimum ratio	8.00%	8.00%
Well capitalized amount	$ 2,694,446	$ 2,648,805
Well capitalized ratio	10.00%	10.00%
Actual amount	$ 3,215,845	$ 3,112,758
Actual ratio	11.94%	11.76%
Capital Conversion Buffer	3.94%