Regulatory Restrictions - Schedule of Capital Ratios (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Old National Bancorp [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total capital to risk-weighted assets Actual, Amount	$ 1,828,312	$ 1,748,231
Common equity Tier 1 capital to risk-weighted assets Actual, Amount	1,706,727	1,617,936
Tier 1 capital to risk-weighted assets Actual, Amount	1,706,727	1,617,936
Tier 1 capital to average assets Actual, Amount	$ 1,706,727	$ 1,617,936
Total capital to risk-weighted assets Actual, Ratio	12.99%	12.27%
Common equity Tier 1 capital to risk-weighted assets Actual, Ratio	12.13%	11.36%
Tier 1 capital to risk-weighted assets Actual, Ratio	12.13%	11.36%
Tier 1 capital to average assets Actual, Ratio	8.88%	9.17%
Total capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	$ 1,477,763	$ 1,496,099
Common equity Tier 1 capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	985,175	997,399
Tier 1 capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	1,196,284	1,211,128
Tier 1 capital to average assets Fully Phased-In Regulatory Guidelines Minimum, Amount	$ 768,537	$ 705,681
Total capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Ratio	10.50%	10.50%
Common equity Tier 1 capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Ratio	7.00%	7.00%
Tier 1 capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Ratio	8.50%	8.50%
Tier 1 capital to average assets Fully Phased-In Regulatory Guidelines Minimum, Ratio	4.00%	4.00%
Old National Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total capital to risk-weighted assets Actual, Amount	$ 1,891,612	$ 1,769,930
Common equity Tier 1 capital to risk-weighted assets Actual, Amount	1,822,337	1,699,945
Tier 1 capital to risk-weighted assets Actual, Amount	1,822,737	1,699,945
Tier 1 capital to average assets Actual, Amount	$ 1,822,737	$ 1,699,945
Total capital to risk-weighted assets Actual, Ratio	13.50%	12.47%
Common equity Tier 1 capital to risk-weighted assets Actual, Ratio	13.01%	11.98%
Tier 1 capital to risk-weighted assets Actual, Ratio	13.01%	11.98%
Tier 1 capital to average assets Actual, Ratio	9.62%	9.58%
Total capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	$ 1,471,122	$ 1,489,938
Common equity Tier 1 capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	980,748	993,292
Tier 1 capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	1,190,909	1,206,141
Tier 1 capital to average assets Fully Phased-In Regulatory Guidelines Minimum, Amount	$ 757,783	$ 709,929
Total capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Ratio	10.50%	10.50%
Common equity Tier 1 capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Ratio	7.00%	7.00%
Tier 1 capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Ratio	8.50%	8.50%
Tier 1 capital to average assets Fully Phased-In Regulatory Guidelines Minimum, Ratio	4.00%	4.00%
Total capital to risk-weighted assets Well Capitalized Guidelines, Amount	$ 1,401,069	$ 1,418,989
Common equity Tier 1 capital to risk-weighted assets Well Capitalized Guidelines, Amount	910,695	922,343
Tier 1 capital to risk-weighted assets Well Capitalized Guidelines, Amount	1,120,855	1,135,191
Tier 1 capital to average assets Well Capitalized Guidelines, Amount	$ 947,228	$ 887,412
Total capital to risk-weighted assets Well Capitalized Guidelines, Ratio	10.00%	10.00%
Common equity Tier 1 capital to risk-weighted assets Well Capitalized Guidelines, Ratio	6.50%	6.50%
Tier 1 capital to risk-weighted assets Well Capitalized Guidelines, Ratio	8.00%	8.00%
Tier 1 capital to average assets Well Capitalized Guidelines, Ratio	5.00%	5.00%