Regulatory Restrictions - Schedule of Capital Ratios (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Old National Bancorp [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total capital to risk-weighted assets Actual, Amount	$ 1,424,123	$ 1,229,878
Common equity Tier 1 capital to risk-weighted assets Actual, Amount	1,298,326	1,162,817
Tier 1 capital to risk-weighted assets Actual, Amount	1,298,326	1,176,849
Tier 1 capital to average assets Actual, Amount	$ 1,298,326	$ 1,176,849
Total capital to risk-weighted assets Actual, Ratio	11.40%	12.18%
Common equity Tier 1 capital to risk-weighted assets Actual, Ratio	10.39%	11.51%
Tier 1 capital to risk-weighted assets Actual, Ratio	10.39%	11.65%
Tier 1 capital to average assets Actual, Ratio	8.28%	8.43%
Total capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	$ 1,311,600	$ 1,060,662
Common equity Tier 1 capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	874,400	707,108
Tier 1 capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	1,061,772	858,631
Tier 1 capital to average assets Fully Phased-In Regulatory Guidelines Minimum, Amount	$ 627,258	$ 558,673
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	$ 1,458,546	$ 1,240,180
Common equity Tier 1 capital to risk-weighted assets Actual, Amount	1,393,059	1,187,151
Tier 1 capital to risk-weighted assets Actual, Amount	1,393,059	1,187,151
Tier 1 capital to average assets Actual, Amount	$ 1,393,059	$ 1,187,151
Total capital to risk-weighted assets Actual, Ratio	11.73%	12.35%
Common equity Tier 1 capital to risk-weighted assets Actual, Ratio	11.21%	11.82%
Tier 1 capital to risk-weighted assets Actual, Ratio	11.21%	11.82%
Tier 1 capital to average assets Actual, Ratio	8.93%	8.55%
Total capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	$ 1,305,076	$ 1,054,305
Common equity Tier 1 capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	870,051	702,870
Tier 1 capital to risk-weighted assets Fully Phased-In Regulatory Guidelines Minimum, Amount	1,056,490	853,485
Tier 1 capital to average assets Fully Phased-In Regulatory Guidelines Minimum, Amount	$ 623,758	$ 555,161
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	$ 1,242,929	$ 1,004,100
Common equity Tier 1 capital to risk-weighted assets Well Capitalized Guidelines, Amount	807,904	652,665
Tier 1 capital to risk-weighted assets Well Capitalized Guidelines, Amount	994,344	803,280
Tier 1 capital to average assets Well Capitalized Guidelines, Amount	$ 779,697	$ 693,951
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%