Goodwill and Intangible Assets - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Goodwill [Line Items]
Goodwill		$ 141	$ 141	$ 120
Goodwill, Acquired During Period			21
Impairment of goodwill		0	0
Amortization expense		14	7
Impairment of technology and intangible assets		0	0
Roosevelt [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets acquired		4
Goodwill [Line Items]
Goodwill, Acquired During Period	$ 21
Servicing
Goodwill [Line Items]
Goodwill		85	85	80
Goodwill, Acquired During Period			5
Servicing | Roosevelt [Member]
Goodwill [Line Items]
Goodwill, Acquired During Period			5
Originations
Goodwill [Line Items]
Goodwill		28	28	28
Goodwill, Acquired During Period			0
Corporate/Other(2)
Goodwill [Line Items]
Goodwill		28	28	$ 12
Goodwill, Acquired During Period			16
Corporate/Other(2) | Roosevelt [Member]
Goodwill [Line Items]
Goodwill, Acquired During Period			$ 16
Flagstar Bank, N.A.
Finite-Lived Intangible Assets [Line Items]
Intangible assets acquired		105
Rushmore
Finite-Lived Intangible Assets [Line Items]
Intangible assets acquired		$ 23