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.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481933/310-10-55-12A

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-34

Reference 3: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 33
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-33

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

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

- Definition

Percentage of decrease in weighted average contractual interest rate of financing receivable modified for debtor experiencing financial difficulty.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModifiedWeightedAverageInterestRateDecreaseFromModification
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Period of increase in weighted average maturity of financing receivable modified for debtor experiencing financial difficulty, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModifiedWeightedAverageTermIncreaseFromModification
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ContractualInterestRateReductionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fbc_MortgageLoansHeldForInvestmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fbc_MultiFamilyLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- 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:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fbc_CommercialRealEstateLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fbc_OneToFourFamilyFirstMortgageLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fbc_CommercialAndIndustrialLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Loans and Leases - Loan Modifications (Details) - Loans and leases held for investment:	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Interest Rate Reduction \| Multi-family \| Minimum
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average contractual interest rate				8.08%
Interest Rate Reduction \| Multi-family \| Maximum
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average contractual interest rate				6.00%
Interest Rate Reduction \| Commercial real estate \| Minimum
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average contractual interest rate	0.00%		0.00%	8.13%
Interest Rate Reduction \| Commercial real estate \| Maximum
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average contractual interest rate	0.00%		0.00%	6.95%
Interest Rate Reduction \| One-to-four family first mortgage \| Minimum
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average contractual interest rate	6.24%	4.76%	6.33%	4.69%
Interest Rate Reduction \| One-to-four family first mortgage \| Maximum
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average contractual interest rate	4.62%	3.65%	4.73%	3.70%
Interest Rate Reduction \| Commercial and Industrial \| Minimum
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average contractual interest rate		8.51%		7.44%
Interest Rate Reduction \| Commercial and Industrial \| Maximum
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average contractual interest rate		6.00%		6.31%
Interest Rate Reduction \| Other \| Minimum
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average contractual interest rate	10.56%		10.58%	10.72%
Interest Rate Reduction \| Other \| Maximum
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average contractual interest rate	6.75%		4.79%	4.34%
Term Extension \| Multi-family
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average Term (in years)				0 years
Term Extension \| Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average Term (in years)	6 months		6 months	0 years
Term Extension \| One-to-four family first mortgage
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average Term (in years)	7 years 4 months 24 days	10 years 7 months 6 days	12 years 2 months 12 days	12 years 2 months 12 days
Term Extension \| Commercial and Industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average Term (in years)		1 year		4 months 24 days
Term Extension \| Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average Term (in years)	12 years 2 months 12 days		7 years 2 months 12 days	1 year 9 months 18 days