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Deposits - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Deposits [Abstract]
Brokered demand deposits	$ 100.0
Brokered time deposits	19.6
Time deposits, meet or exceed FDIC insurance limit of $250,000	$ 285.1	$ 546.0

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- Definition

Interest-bearing Domestic Deposit, Brokered, Demand

+ References

No definition available.

+ Details

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X

- Definition

Interest-bearing Domestic Deposit, Brokered, Time Deposits

+ References

No definition available.

+ Details

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Namespace Prefix:	onb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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- Definition

Amount of time deposit liabilities, including certificates of deposit, in denominations that meet or exceed the Federal Deposit Insurance Corporation (FDIC) insurance limit.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 405
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=116652737&loc=d3e64164-112818

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