Basis of Presentation (Details) $ in Millions	Apr. 26, 2021 USD ($) productionOffice branch state shares	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Business Acquisition [Line Items]
Assets | $	$ 87,000	$ 27,042	$ 31,038
Number of traditional branches | branch	400
Number of states in which branches are located	9
Number of loan production offices | productionOffice	84
Number of states in which offices are located	28
NYCB
Business Acquisition [Line Items]
Common stock portion, number of Flagstar stock for each share of NYCB common stock | shares	4.0151