Regulatory Restrictions - Schedule of Capital Ratios (Detail) $ in Thousands	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Minimum | Basel III Capital Rules
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 capital to average assets Fully Phased-In Regulatory Guidelines Minimum, Ratio	0.040
Old National Bancorp
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total capital to risk-weighted assets Actual, Amount	$ 1,949,757	$ 1,828,312
Common equity Tier 1 capital to risk-weighted assets Actual, Amount	1,805,194	1,706,727
Tier 1 capital to risk-weighted assets Actual, Amount	1,805,194	1,706,727
Tier 1 capital to average assets Actual, Amount	$ 1,805,194	$ 1,706,727
Total capital to risk-weighted assets Actual, Ratio	0.1269	0.1299
Common equity Tier 1 capital to risk-weighted assets Actual, Ratio	11.75%	12.13%
Tier 1 capital to risk-weighted assets Actual, Ratio	0.1175	0.1213
Tier 1 capital to average assets Actual, Ratio	0.0820	0.0888
Total capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	$ 1,613,753	$ 1,477,763
Common equity Tier 1 capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	1,075,835	985,175
Tier 1 capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	1,306,371	1,196,284
Tier 1 capital to average assets Fully Phased-In Regulatory Guidelines Minimum, Amount	$ 880,845	$ 768,537
Total capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Ratio	0.1050	0.1050
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	0.0850	0.0850
Tier 1 capital to average assets Fully Phased-In Regulatory Guidelines Minimum, Ratio	0.0400	0.0400
Old National Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total capital to risk-weighted assets Actual, Amount	$ 1,973,180	$ 1,891,612
Common equity Tier 1 capital to risk-weighted assets Actual, Amount	1,870,617	1,822,337
Tier 1 capital to risk-weighted assets Actual, Amount	1,870,617	1,822,737
Tier 1 capital to average assets Actual, Amount	$ 1,870,617	$ 1,822,737
Total capital to risk-weighted assets Actual, Ratio	0.1290	0.1350
Common equity Tier 1 capital to risk-weighted assets Actual, Ratio	12.23%	13.01%
Tier 1 capital to risk-weighted assets Actual, Ratio	0.1223	0.1301
Tier 1 capital to average assets Actual, Ratio	0.0867	0.0962
Total capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	$ 1,606,657	$ 1,471,122
Common equity Tier 1 capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	1,071,104	980,748
Tier 1 capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	1,300,627	1,190,909
Tier 1 capital to average assets Fully Phased-In Regulatory Guidelines Minimum, Amount	$ 863,087	$ 757,783
Total capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Ratio	0.1050	0.1050
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	0.0850	0.0850
Tier 1 capital to average assets Fully Phased-In Regulatory Guidelines Minimum, Ratio	0.0400	0.0400
Total capital to risk-weighted assets Well Capitalized Guidelines, Amount	$ 1,530,149	$ 1,401,069
Common equity Tier 1 capital to risk-weighted assets Well Capitalized Guidelines, Amount	994,597	910,695
Tier 1 capital to risk-weighted assets Well Capitalized Guidelines, Amount	1,224,119	1,120,855
Tier 1 capital to average assets Well Capitalized Guidelines, Amount	$ 1,078,859	$ 947,228
Total capital to risk-weighted assets Well Capitalized Guidelines, Ratio	0.1000	0.1000
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	0.0800	0.0800
Tier 1 capital to average assets Well Capitalized Guidelines, Ratio	0.0500	0.0500