Mortgage Servicing Rights and Related Liabilities - Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Servicing Asset at Amortized Cost [Line Items]
Excess spread financing - fair value	$ 822	$ 934
Mortgage servicing rights financing - fair value	20	$ 33
Forward MSRs Sold
Servicing Asset at Amortized Cost [Line Items]
Principal Amount Outstanding on Loans Managed and Securitized or Asset-backed Financing Arrangement	1,226		$ 94
Forward MSRs Sold, Subservicing Retained
Servicing Asset at Amortized Cost [Line Items]
Principal Amount Outstanding on Loans Managed and Securitized or Asset-backed Financing Arrangement	$ 1,144		$ 0