Share-Based Compensation (Detail) - USD ($) shares in Thousands	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Remaining shares available for issuance (in shares)	2,500
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted during period (in shares)	100
Share-based compensation awards, vesting period	36 months
Nonvested shares outstanding (in shares)	500
Unrecognized compensation expense	$ 6,200,000
Expected weighted-average period for cost recognition	2 years 1 month 6 days
Share-based compensation expense	$ 700,000	$ 700,000
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted during period (in shares)	200
Share-based compensation awards, vesting period	36 months
Nonvested shares outstanding (in shares)	900
Unrecognized compensation expense	$ 6,400,000
Expected weighted-average period for cost recognition	2 years 2 months 12 days
Share-based compensation expense	$ 600,000	$ 1,400,000
Stock Options \| Old National Bancorp
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Incremental expense associated with conversion of stock appreciation rights	$ 0
Stock Appreciation Rights (SARs) \| Old National Bancorp
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested shares outstanding (in shares)	28

Amount, after tax, of expense for award under share-based payment arrangement.

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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-Publisher FASB
-Name Accounting Standards Codification
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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Number of shares authorized for issuance under share-based payment arrangement.

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An excess of the fair value of the modified award over the fair value of the award immediately before the modification.

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