Cover Page - shares	6 Months Ended
	Jun. 30, 2022	Aug. 05, 2022
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2022
Document Transition Report	false
Entity File Number	001-16577
Entity Registrant Name	Flagstar Bancorp, Inc.
Entity Incorporation, State or Country Code	MI
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	48098-2639
City Area Code	248
Local Phone Number	312-2000
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
Title of 12(b) Security	Common stock
Trading Symbol	FBC
Security Exchange Name	NYSE
Entity Common Stock, Shares Outstanding		53,330,138
Document Fiscal Period Focus	Q2
Amendment Flag	false
Document Fiscal Year Focus	2022
Current Fiscal Year End Date	--12-31
Entity Central Index Key	0001033012