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Share-Based Compensation and Other Employee Benefit Plans - Additional Information (Detail) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2019 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Remaining shares available for issuance (in shares) |  | 2,900,000 |  |
Stock based compensation expense |  | $ 7,700,000 | $ 8,000,000.0 | $ 8,100,000
Total income tax benefit, stock-based compensation cost |  | 1,900,000 | 2,000,000.0 | 2,000,000.0
Discretionary profit sharing |  | $ 0 | $ 0 | 0
Shares allocated to the employee stock ownership plan (in shares) |  | 600,000 | 600,000 |
Contribution expense under employee stock ownership plan |  | $ 9,500,000 | $ 9,800,000 | 8,600,000
Employer Matching Contribution Tranche One |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Contribution plan, employer matching contribution percentage | 75.00% |  |  |
Defined contribution plans employee contribution percentage of eligible compensation matched by employer | 4.00% |  |  |
Employer Matching Contribution Tranche Two |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Contribution plan, employer matching contribution percentage | 50.00% |  |  |
Defined contribution plans employee contribution percentage of eligible compensation matched by employer | 4.00% |  |  |
Restricted Stock |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Share-based compensation awards, vesting period |  | 3 years |  |
Unrecognized compensation expense |  | $ 5,700,000 |  |
Expected weighted-average period for cost recognition (in years) |  | 2 years 1 month 6 days |  |
Total fair value of shares vested |  | $ 2,900,000 | $ 3,400,000 | $ 3,400,000
Restricted Stock Units |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Share-based compensation awards, vesting period |  | 3 years |  |
Unrecognized compensation expense |  | $ 3,600,000 |  |
Expected weighted-average period for cost recognition (in years) |  | 1 year 8 months 12 days |  |
Stock Options |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Contractual term, in years |  | 10 years |  |
Stock Options | Old National Bancorp |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Incremental expense associated with conversion of stock awards |  | $ 0 |  |
Stock Options | Minimum |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Share-based compensation awards, vesting period |  | 3 years |  |
Stock Options | Maximum |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Share-based compensation awards, vesting period |  | 5 years |  |
Outside Director Stock Compensation Program | Amended and Restated 2008 Incentive Compensation Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Shares issued (in shares) |  | 28,000 | 12,000 | 16,000

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- Definition Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(i) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.D.2) -URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809 + Details
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