Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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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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Share-Based Compensation - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Remaining shares available for issuance	3,700,000
Stock based compensation expense	$ 8,000,000.0	$ 8,100,000	$ 6,300,000
Total income tax benefit, stock-based compensation cost	$ 2,000,000.0	2,000,000.0	2,400,000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation awards, vesting period	3 years
Unrecognized compensation expense	$ 4,200,000
Expected weighted-average period for cost recognition (in years)	1 year 9 months 18 days
Total fair value of shares vested	$ 3,400,000	$ 3,400,000	$ 3,400,000
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation awards, vesting period	3 years
Unrecognized compensation expense	$ 4,100,000
Expected weighted-average period for cost recognition (in years)	1 year 8 months 12 days
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Contractual term, in years	10 years
Stock Options [Member] \| Old National Bancorp [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Incremental expense associated with conversion of stock awards	$ 0
Stock Options [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation awards, vesting period	3 years
Stock Options [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation awards, vesting period	5 years
Outside Director Stock Compensation Program [Member] \| Amended and Restated 2008 Incentive Compensation Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares, issued	12,000	16,000	20,000