Fair Value Measurements - Fair Value by Balance Sheet Line Item (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Financial assets
Restricted cash	$ 118	$ 135	$ 152
Loans subject to repurchase from Ginnie Mae	2,703	6,159
Financial liabilities
Unsecured senior notes, net	2,076	2,074
Advance and warehouse facilities, net	8,206	6,258
Recurring Fair Value Measurements
Financial assets
Cash and cash equivalents	731	695
Restricted cash	118	135
Loans subject to repurchase from Ginnie Mae	2,703	6,159
Financial liabilities
Unsecured senior notes, net	2,076	2,074
Advance and warehouse facilities, net	8,206	6,258
Liability for loans subject to repurchase from Ginnie Mae	2,703	6,159
Recurring Fair Value Measurements | Level 1
Financial assets
Cash and cash equivalents	731	695
Restricted cash	118	135
Loans subject to repurchase from Ginnie Mae	0	0
Financial liabilities
Unsecured senior notes, net	2,168	2,208
Advance and warehouse facilities, net	0	0
Liability for loans subject to repurchase from Ginnie Mae	0	0
Recurring Fair Value Measurements | Level 2
Financial assets
Cash and cash equivalents	0	0
Restricted cash	0	0
Loans subject to repurchase from Ginnie Mae	2,703	6,159
Financial liabilities
Unsecured senior notes, net	0	0
Advance and warehouse facilities, net	8,219	6,269
Liability for loans subject to repurchase from Ginnie Mae	2,703	6,159
Recurring Fair Value Measurements | Level 3
Financial assets
Cash and cash equivalents	0	0
Restricted cash	0	0
Loans subject to repurchase from Ginnie Mae	0	0
Financial liabilities
Unsecured senior notes, net	0	0
Advance and warehouse facilities, net	0	0
Liability for loans subject to repurchase from Ginnie Mae	0	0
Nonrecurring Fair Value Measurements
Financial assets
Advances and other receivables, net	909	940
Nonrecurring Fair Value Measurements | Level 1
Financial assets
Advances and other receivables, net	0	0
Nonrecurring Fair Value Measurements | Level 2
Financial assets
Advances and other receivables, net	0	0
Nonrecurring Fair Value Measurements | Level 3
Financial assets
Advances and other receivables, net	$ 909	$ 940