Mortgage Servicing Rights - Summary of Adverse Changes to Weighted-Average Assumptions on the Fair Value of Servicing Rights (Details) - Mortgage servicing rights $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2019 USD ($) $ / loan	Dec. 31, 2018 USD ($) $ / loan
Actual
Option adjusted spread	5.51%	5.42%
Constant prepayment rate	10.17%	9.57%
Weighted average cost to service per loan (in dollars per share) \| $ / loan	86.11	85.57
Fair value impact due to 10% adverse change
Option adjusted spread	$ 270	$ 284
Constant prepayment rate	260	278
Weighted average cost to service per loan (in dollars per share)	272	286
Fair value impact due to 20% adverse change
Option adjusted spread	266	280
Constant prepayment rate	247	268
Weighted average cost to service per loan (in dollars per share)	$ 269	$ 283

Sensitivity Analysis, Actual [Abstract]

No definition available.

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

- Definition

Sensitivity Analysis, Adverse Change, Tranche 1 [Abstract]

+ References

No definition available.

+ Details

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

- Definition

Sensitivity Analysis, Adverse Change, Tranche 2 [Abstract]

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

Name:	fbc_SensitivityAnalysisofFairValueTransferorsInterestsinTransferredFinancialAssetsImpactofOtherthan10or20PercentAdverseChangeinOptionAdjustedSpreadPercent
Namespace Prefix:	fbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	fbc_SensitivityAnalysisofFairValueTransferorsInterestsinTransferredFinancialAssetsImpactofOtherthan10or20PercentAdverseChangeinOtherAssumptionPerLoan
Namespace Prefix:	fbc_
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	instant

- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=109247956&loc=d3e107314-111719

+ Details

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

- Definition

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.

+ References

+ Details

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

- Definition

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.

+ References

+ Details

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

- Definition

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.

+ References

+ Details

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

- Definition

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).

+ References

+ Details

Name:	us-gaap_SensitivityAnalysisOfFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesImpactOfOtherThan10Or20PercentAdverseChangeInPrepaymentSpeedPercent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_ManagingRisksInherentInServicingAssetsAndServicingLiabilitiesByTypeOfRiskAxis=fbc_ResidentialMortgageServicingRightsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: