| Document and Entity Information | Jul. 29, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Jul. 28, 2021 |
| Entity Registrant Name | Flagstar Bancorp, Inc. |
| Entity Central Index Key | 0001033012 |
| Entity Incorporation, State or Country Code | MI |
| Entity File Number | 1-16577 |
| Entity Tax Identification Number | 38-3150651 |
| Entity Address, Address Line One | 5151 Corporate Drive, |
| Entity Address, City or Town | Troy, |
| Entity Address, State or Province | MI |
| Entity Address, Postal Zip Code | 48098 |
| City Area Code | 248 |
| Local Phone Number | 312-2000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Title of 12(b) Security | Common stock |
| Trading Symbol | FBC |
| Security Exchange Name | NYSE |
