| Document and Entity Information | Apr. 26, 2021 |
| Cover [Abstract] | |
| Entity Registrant Name | FLAGSTAR BANCORP INC |
| Amendment Flag | false |
| Entity Central Index Key | 0001033012 |
| Document Type | 8-K |
| Document Period End Date | Apr. 26, 2021 |
| Entity Incorporation State 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 | true |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common stock |
| Trading Symbol | FBC |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
