BORROWINGS - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022
Federal Home Loan Bank, Advances [Line Items]
Fixed rate, term borrowings from FHLB	$ 397.5	$ 310.5
Investment securities and loans pledged to secure advances	$ 717.5	$ 638.6
Weighted average fixed rate	4.90%	3.87%
Minimum
Federal Home Loan Bank, Advances [Line Items]
Remaining maturities on advances	1 month
Maximum
Federal Home Loan Bank, Advances [Line Items]
Remaining maturities on advances	30 months

Federal Home Loan Bank, Advances, Remaining Term

No definition available.

Name:	blue_FederalHomeLoanBankAdvancesRemainingTerm
Namespace Prefix:	blue_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of advances from the Federal Home Loan Bank (FHLB).

+ References

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankAmountOfAdvancesByBranch
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of the collateral pledged in support of federal home loan bank advances outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 470
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480876/942-470-45-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480848/942-470-50-3

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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.

+ References

No definition available.

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvancesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average interest rate on advances made and reported by Federal Home Loan Bank (FHLBank).

+ References

No definition available.

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvancesWeightedAverageInterestRate
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: