Regulatory Restrictions - Schedule of Capital Ratios (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Old National Bancorp [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total capital to risk-weighted assets Actual, Amount	$ 1,748,231	$ 1,424,123
Common equity Tier 1 capital to risk-weighted assets Actual, Amount	1,617,936	1,298,327
Tier 1 capital to risk-weighted assets Actual, Amount	1,617,936	1,298,327
Tier 1 capital to average assets Actual, Amount	$ 1,617,936	$ 1,298,327
Total capital to risk-weighted assets Actual, Ratio	12.27%	11.40%
Common equity Tier 1 capital to risk-weighted assets Actual, Ratio	11.36%	10.39%
Tier 1 capital to risk-weighted assets Actual, Ratio	11.36%	10.39%
Tier 1 capital to average assets Actual, Ratio	9.17%	8.28%
Total capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	$ 1,496,099	$ 1,311,600
Common equity Tier 1 capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	997,399	874,400
Tier 1 capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	1,211,128	1,061,772
Tier 1 capital to average assets Fully Phased-In Regulatory Guidelines Minimum, Amount	$ 705,681	$ 627,258
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	$ 1,769,930	$ 1,458,546
Common equity Tier 1 capital to risk-weighted assets Actual, Amount	1,699,945	1,393,059
Tier 1 capital to risk-weighted assets Actual, Amount	1,699,945	1,393,059
Tier 1 capital to average assets Actual, Amount	$ 1,699,945	$ 1,393,059
Total capital to risk-weighted assets Actual, Ratio	12.47%	11.73%
Common equity Tier 1 capital to risk-weighted assets Actual, Ratio	11.98%	11.21%
Tier 1 capital to risk-weighted assets Actual, Ratio	11.98%	11.21%
Tier 1 capital to average assets Actual, Ratio	9.58%	8.93%
Total capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	$ 1,489,938	$ 1,305,076
Common equity Tier 1 capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	993,292	870,051
Tier 1 capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	1,206,141	1,056,490
Tier 1 capital to average assets Fully Phased-In Regulatory Guidelines Minimum, Amount	$ 709,929	$ 623,758
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	$ 1,418,989	$ 1,242,929
Common equity Tier 1 capital to risk-weighted assets Well Capitalized Guidelines, Amount	922,343	807,904
Tier 1 capital to risk-weighted assets Well Capitalized Guidelines, Amount	1,135,191	994,344
Tier 1 capital to average assets Well Capitalized Guidelines, Amount	$ 887,412	$ 779,697
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%