SIGNIFICANT ACCOUNTING POLICIES - Trust Assets (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Accounting Policies [Abstract]
Assets under care	$ 15,660	$ 13,830

Assets under care held for customers, that are not included in the Company's consolidated financial statements.

No definition available.

Name:	buse_CustomerAssetsHeldInTrust
Namespace Prefix:	buse_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant