Fair Value Measurements - Fair Value by Balance Sheet Line Item (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Financial assets
Reverse mortgage interests, net	$ 5,091	$ 5,253
Loans subject to repurchase from Ginnie Mae	5,816	6,159
Financial liabilities
Unsecured senior notes, net	2,074	2,074
Advance and warehouse facilities, net	7,379	6,763
Other nonrecourse debt	4,221	4,424
Recurring Fair Value Measurements
Financial assets
Cash and cash equivalents	674	695
Restricted cash	261	218
Loans subject to repurchase from Ginnie Mae	5,816	6,159
Financial liabilities
Unsecured senior notes, net	2,074	2,074
Advance and warehouse facilities, net	7,379	6,763
Liability for loans subject to repurchase from Ginnie Mae	5,816	6,159
Recurring Fair Value Measurements \| Level 1
Financial assets
Cash and cash equivalents	674	695
Restricted cash	261	218
Loans subject to repurchase from Ginnie Mae	0	0
Financial liabilities
Unsecured senior notes, net	2,120	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	5,816	6,159
Financial liabilities
Unsecured senior notes, net	0	0
Advance and warehouse facilities, net	7,388	6,774
Liability for loans subject to repurchase from Ginnie Mae	5,816	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	838	940
Reverse mortgage interests, net	5,091	5,253
Nonrecurring Fair Value Measurements \| Participating interest financing
Financial liabilities
Other nonrecourse debt	3,318	3,485
Nonrecurring Fair Value Measurements \| Trust 2020-1
Financial liabilities
Other nonrecourse debt	469	488
Nonrecurring Fair Value Measurements \| Trust 2019-2
Financial liabilities
Other nonrecourse debt	231	240
Nonrecurring Fair Value Measurements \| Trust 2019-1
Financial liabilities
Other nonrecourse debt	203	211
Nonrecurring Fair Value Measurements \| Level 1
Financial assets
Advances and other receivables, net	0	0
Reverse mortgage interests, net	0	0
Nonrecurring Fair Value Measurements \| Level 1 \| Participating interest financing
Financial liabilities
Other nonrecourse debt	0	0
Nonrecurring Fair Value Measurements \| Level 1 \| Trust 2020-1
Financial liabilities
Other nonrecourse debt	0	0
Nonrecurring Fair Value Measurements \| Level 1 \| Trust 2019-2
Financial liabilities
Other nonrecourse debt	0	0
Nonrecurring Fair Value Measurements \| Level 1 \| Trust 2019-1
Financial liabilities
Other nonrecourse debt	0	0
Nonrecurring Fair Value Measurements \| Level 2
Financial assets
Advances and other receivables, net	0	0
Reverse mortgage interests, net	0	0
Nonrecurring Fair Value Measurements \| Level 2 \| Participating interest financing
Financial liabilities
Other nonrecourse debt	0	0
Nonrecurring Fair Value Measurements \| Level 2 \| Trust 2020-1
Financial liabilities
Other nonrecourse debt	0	0
Nonrecurring Fair Value Measurements \| Level 2 \| Trust 2019-2
Financial liabilities
Other nonrecourse debt	0	0
Nonrecurring Fair Value Measurements \| Level 2 \| Trust 2019-1
Financial liabilities
Other nonrecourse debt	0	0
Nonrecurring Fair Value Measurements \| Level 3
Financial assets
Advances and other receivables, net	838	940
Reverse mortgage interests, net	5,166	5,383
Nonrecurring Fair Value Measurements \| Level 3 \| Participating interest financing
Financial liabilities
Other nonrecourse debt	3,319	3,496
Nonrecurring Fair Value Measurements \| Level 3 \| Trust 2020-1
Financial liabilities
Other nonrecourse debt	471	490
Nonrecurring Fair Value Measurements \| Level 3 \| Trust 2019-2
Financial liabilities
Other nonrecourse debt	231	241
Nonrecurring Fair Value Measurements \| Level 3 \| Trust 2019-1
Financial liabilities
Other nonrecourse debt	$ 203	$ 212

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