| Document and Entity Information | Dec. 11, 2024 |
| Document And Entity Information [Line Items] | |
| Amendment Flag | false |
| Entity Central Index Key | 0000910073 |
| Document Type | 8-K |
| Document Period End Date | Dec. 11, 2024 |
| Entity Registrant Name | FLAGSTAR FINANCIAL, INC. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 1-31565 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Common Stock [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Common Stock, $0.01 par value per share |
| Trading Symbol | FLG |
| Security Exchange Name | NYSE |
| Bifurcated Option Notes Unit Securities [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Bifurcated Option Note Unit SecuritiES SM |
| Trading Symbol | FLG RPU |
| Security Exchange Name | NYSE |
| Fixed To Floating Rate Series A Noncumulative Perpetual Preferred Stock [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Fixed-to-Floating Rate Series A Noncumulative Perpetual Preferred Stock, $0.01 par value |
| Trading Symbol | FLG RPA |
| Security Exchange Name | NYSE |
