Sensitivity Analysis, Actual [Abstract]

No definition available.

Name:	fbc_SensitivityAnalysisActualAbstract
Namespace Prefix:	fbc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Sensitivity Analysis, Adverse Change, Tranche 1 [Abstract]

No definition available.

Name:	fbc_SensitivityAnalysisAdverseChangeTranche1Abstract
Namespace Prefix:	fbc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Sensitivity Analysis, Adverse Change, Tranche 2 [Abstract]

No definition available.

Name:	fbc_SensitivityAnalysisAdverseChangeTranche2Abstract
Namespace Prefix:	fbc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Sensitivity Analysis of Fair Values, Transferor's Interests in Transferred Financial Assets, Impact of Other than 10 or 20 Percent Adverse Change in Other Assumption, Per Loan

No definition available.

Name:	fbc_SensitivityAnalysisOfFairValuesTransferorsInterestsInTransferredFinancialAssetsImpactOfOtherThan10Or20PercentAdverseChangeInOtherAssumptionPerLoan
Namespace Prefix:	fbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Sensitivity Analysis of Fair Value, Transferor's Interests in Transferred Financial Assets, Impact of 10 Percent Adverse Change in Option Adjusted Spread

No definition available.

Name:	fbc_SensitivityAnalysisofFairValueTransferorsInterestsinTransferredFinancialAssetsImpactof10PercentAdverseChangeinOptionAdjustedSpread
Namespace Prefix:	fbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Sensitivity Analysis of Fair Value, Transferor's Interests in Transferred Financial Assets, Impact of 20 Percent Adverse Change in Option Adjusted Spread

No definition available.

Name:	fbc_SensitivityAnalysisofFairValueTransferorsInterestsinTransferredFinancialAssetsImpactof20PercentAdverseChangeinOptionAdjustedSpread
Namespace Prefix:	fbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Sensitivity Analysis of Fair Value, Transferor's Interests in Transferred Financial Assets, Impact of Other than 10 or 20 Percent Adverse Change in Option Adjusted Spread, Percent

No definition available.

Name:	fbc_SensitivityAnalysisofFairValueTransferorsInterestsinTransferredFinancialAssetsImpactofOtherthan10or20PercentAdverseChangeinOptionAdjustedSpreadPercent
Namespace Prefix:	fbc_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	duration

The hypothetical financial impact of a 10 percent adverse change of the specified assumption on the fair value of transferor's interests in transferred financial assets (including any servicing assets or servicing liabilities) as of the balance sheet date.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 860
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481326/860-20-50-4

Name:	us-gaap_SensitivityAnalysisOfFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesImpactOf10PercentAdverseChangeInOtherAssumption
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The hypothetical financial impact of a 10 percent adverse change of prepayment speed on the fair value of transferor's interests in transferred financial assets (including any servicing assets or servicing liabilities) as of the balance sheet date.

Name:	us-gaap_SensitivityAnalysisOfFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesImpactOf10PercentAdverseChangeInPrepaymentSpeed
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The hypothetical financial impact of a 20 percent adverse change of the specified assumption on the fair value of transferor's interests in transferred financial assets (including any servicing assets or servicing liabilities) as of the balance sheet date.

Name:	us-gaap_SensitivityAnalysisOfFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesImpactOf20PercentAdverseChangeInOtherAssumption
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The hypothetical financial impact of a 20 percent adverse change of prepayment speed on the fair value of transferor's interests in transferred financial assets (including any servicing assets or servicing liabilities) as of the balance sheet date.

Name:	us-gaap_SensitivityAnalysisOfFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesImpactOf20PercentAdverseChangeInPrepaymentSpeed
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

A specified percentage variance in the prepayment speed assumption in the determination of fair value of transferor's interests in transferred financial assets (including any servicing assets or servicing liabilities).