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Other Borrowings - Additional Information (Detail) - USD ($) $ in Thousands | Nov. 01, 2017 | Mar. 31, 2021 | Dec. 31, 2020 | Aug. 31, 2014
--- | --- | --- | --- | ---
Debt Instrument [Line Items] |  |  |  |
Lease liability |  | $ 11,561 | $ 11,813 |
Old National Bank |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Lease liability |  | 11,561 | 11,813 |
Subordinated Debentures | Anchor Bank (MN) | Subordinated Fixed-to-Floating Notes |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Fixed rate | 5.75% |  |  |
Value of subordinated fixed-to-floating notes assumed | $ 12,000 |  |  |
Subordinated Debentures | Anchor Bank (MN) | Subordinated Fixed-to-Floating Notes | LIBOR |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Variable rate | 4.356% |  |  |
Old National Bancorp | Senior Unsecured Notes |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Senior unsecured notes |  | $ 175,000 | $ 175,000 | $ 175,000
Fixed rate |  | 4.125% |  | 4.125%

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