Cover - shares	6 Months Ended
	Jun. 30, 2025	Jul. 31, 2025
Document Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2025
Document Transition Report	false
Entity File Number	001-31565
Entity Registrant Name	Flagstar Financial, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	06-1377322
Entity Address, Address Line One	102 Duffy Avenue,
Entity Address, City or Town	Hicksville,
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	11801
City Area Code	516
Local Phone Number	683-4100
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		415,574,267
Entity Central Index Key	0000910073
Amendment Flag	false
Document Fiscal Year Focus	2025
Document Fiscal Period Focus	Q2
Current Fiscal Year End Date	--12-31
Common Stock (Par Value: $0.01)
Document Information [Line Items]
Title of 12(b) Security	Common Stock, $0.01 par value per share
Trading Symbol	FLG
Security Exchange Name	NYSE
Bifurcated Option Note Unit Securities
Document Information [Line Items]
Title of 12(b) Security	Bifurcated Option Note Unit SecuritiesSM
Trading Symbol	FLG PRU
Security Exchange Name	NYSE
Depositary Shares each representing a 1/40th interest in a share of Fixed-to-Floating Rate Series A Noncumulative Perpetual Preferred Stock
Document Information [Line Items]
Title of 12(b) Security	Depositary Shares each representing a 1/40th interest in a share of Fixed-to-Floating Rate Series A Noncumulative Perpetual Preferred Stock
Trading Symbol	FLG PRA
Security Exchange Name	NYSE