Fair Value Measurements - Measured on a Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Assets
Mortgage loans held for sale	$ 3,817	$ 4,077
Forward mortgage servicing rights	2,663	3,496
Liabilities
Mortgage servicing rights financing	47	37
Recurring Fair Value Measurements
Assets
Mortgage loans held for sale	3,817	4,077
Forward mortgage servicing rights	2,663	3,496
Liabilities
Mortgage servicing rights financing	47	37
Excess spread financing	1,044	1,311
Recurring Fair Value Measurements | IRLCs
Assets
Derivative asset, fair value	414	135
Recurring Fair Value Measurements | Forward MBS trades
Assets
Derivative asset, fair value	23	7
Liabilities
Derivative financial instruments	42	12
Recurring Fair Value Measurements | LPCs
Assets
Derivative asset, fair value	38	12
Liabilities
Derivative financial instruments	2	3
Recurring Fair Value Measurements | Level 1
Assets
Mortgage loans held for sale	0	0
Forward mortgage servicing rights	0	0
Liabilities
Mortgage servicing rights financing	0	0
Excess spread financing	0	0
Recurring Fair Value Measurements | Level 1 | IRLCs
Assets
Derivative asset, fair value	0	0
Recurring Fair Value Measurements | Level 1 | Forward MBS trades
Assets
Derivative asset, fair value	0	0
Liabilities
Derivative financial instruments	0	0
Recurring Fair Value Measurements | Level 1 | LPCs
Assets
Derivative asset, fair value	0	0
Liabilities
Derivative financial instruments	0	0
Recurring Fair Value Measurements | Level 2
Assets
Mortgage loans held for sale	3,817	4,077
Forward mortgage servicing rights	0	0
Liabilities
Mortgage servicing rights financing	0	0
Excess spread financing	0	0
Recurring Fair Value Measurements | Level 2 | IRLCs
Assets
Derivative asset, fair value	0	135
Recurring Fair Value Measurements | Level 2 | Forward MBS trades
Assets
Derivative asset, fair value	23	7
Liabilities
Derivative financial instruments	42	12
Recurring Fair Value Measurements | Level 2 | LPCs
Assets
Derivative asset, fair value	0	12
Liabilities
Derivative financial instruments	0	3
Recurring Fair Value Measurements | Level 3
Assets
Mortgage loans held for sale	0	0
Forward mortgage servicing rights	2,663	3,496
Liabilities
Mortgage servicing rights financing	47	37
Excess spread financing	1,044	1,311
Recurring Fair Value Measurements | Level 3 | IRLCs
Assets
Derivative asset, fair value	414	0
Recurring Fair Value Measurements | Level 3 | Forward MBS trades
Assets
Derivative asset, fair value	0	0
Liabilities
Derivative financial instruments	0	0
Recurring Fair Value Measurements | Level 3 | LPCs
Assets
Derivative asset, fair value	38	0
Liabilities
Derivative financial instruments	$ 2	$ 0