Fair Value Measurements - Fair Value by Balance Sheet Line Item (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Financial assets
Restricted cash	$ 339,000,000	$ 319,000,000
Reverse mortgage interests, net	7,489,000,000	7,934,000,000
Mortgage loans held for sale	2,170,000,000	1,631,000,000
Mortgage loans held for investment, net	118,000,000	119,000,000
Financial liabilities
Unsecured senior notes	2,461,000,000	2,459,000,000
Advance facilities	578,000,000	595,000,000
Warehouse facilities	3,050,000,000	2,349,000,000
Mortgage servicing rights financing liability	34,000,000	32,000,000
Excess spread financing	1,309,000,000	1,184,000,000
Other nonrecourse debt	6,388,000,000	6,795,000,000
Participating Interest Financing
Financial liabilities
Other nonrecourse debt	5,319,000,000	5,607,000,000
Trust 2017-2
Financial liabilities
Other nonrecourse debt	207,000,000	231,000,000
Trust 2018-1
Financial liabilities
Other nonrecourse debt	252,000,000	284,000,000
Trust 2018-2
Financial liabilities
Other nonrecourse debt	213,000,000	250,000,000
Trust 2018-3
Financial liabilities
Other nonrecourse debt	312,000,000	326,000,000
Legacy Asset
Financial liabilities
Other nonrecourse debt	26,000,000	29,000,000
Recurring Fair Value Measurements
Financial assets
Cash and cash equivalents	181,000,000	242,000,000
Restricted cash	339,000,000	319,000,000
Advances and other receivables, net	1,147,000,000	1,194,000,000
Reverse mortgage interests, net	7,489,000,000	7,934,000,000
Mortgage loans held for sale	2,170,200,000	1,630,800,000
Mortgage loans held for investment, net	117,800,000	119,100,000
Derivative financial instruments	72,000,000	49,000,000
Financial liabilities
Unsecured senior notes	2,461,000,000	2,459,000,000
Advance facilities	578,000,000	595,000,000
Warehouse facilities	3,050,000,000	2,349,000,000
Mortgage servicing rights financing liability	33,700,000	31,700,000
Excess spread financing	1,309,000,000	1,184,000,000
Derivative financial instruments at fair value	22,000,000	20,000,000
Recurring Fair Value Measurements \| Participating Interest Financing
Financial liabilities
Other nonrecourse debt	5,378,000,000	5,675,000,000
Recurring Fair Value Measurements \| Trust 2017-1
Financial liabilities
Other nonrecourse debt	207,000,000
Recurring Fair Value Measurements \| Trust 2017-2
Financial liabilities
Other nonrecourse debt	252,000,000	231,000,000
Recurring Fair Value Measurements \| Trust 2018-1
Financial liabilities
Other nonrecourse debt	213,000,000	284,000,000
Recurring Fair Value Measurements \| Trust 2018-2
Financial liabilities
Other nonrecourse debt	312,000,000	250,000,000
Recurring Fair Value Measurements \| Trust 2018-3
Financial liabilities
Other nonrecourse debt		326,000,000
Recurring Fair Value Measurements \| Legacy Asset
Financial liabilities
Other nonrecourse debt	26,000,000	29,000,000
Recurring Fair Value Measurements \| Level 1
Financial assets
Cash and cash equivalents	181,000,000	242,000,000
Restricted cash	339,000,000	319,000,000
Advances and other receivables, net	0	0
Reverse mortgage interests, net	0	0
Mortgage loans held for sale	0	0
Mortgage loans held for investment, net	0	0
Derivative financial instruments	0	0
Financial liabilities
Unsecured senior notes	2,516,000,000	2,451,000,000
Advance facilities	0	0
Warehouse facilities	0	0
Mortgage servicing rights financing liability	0	0
Excess spread financing	0	0
Derivative financial instruments at fair value	0	0
Recurring Fair Value Measurements \| Level 1 \| Participating Interest Financing
Financial liabilities
Other nonrecourse debt	0	0
Recurring Fair Value Measurements \| Level 1 \| Trust 2017-1
Financial liabilities
Other nonrecourse debt	0
Recurring Fair Value Measurements \| Level 1 \| Trust 2017-2
Financial liabilities
Other nonrecourse debt	0	0
Recurring Fair Value Measurements \| Level 1 \| Trust 2018-1
Financial liabilities
Other nonrecourse debt	0	0
Recurring Fair Value Measurements \| Level 1 \| Trust 2018-2
Financial liabilities
Other nonrecourse debt	0	0
Recurring Fair Value Measurements \| Level 1 \| Trust 2018-3
Financial liabilities
Other nonrecourse debt		0
Recurring Fair Value Measurements \| Level 1 \| Legacy Asset
Financial liabilities
Other nonrecourse debt	0	0
Recurring Fair Value Measurements \| Level 2
Financial assets
Cash and cash equivalents	0	0
Restricted cash	0	0
Advances and other receivables, net	0	0
Reverse mortgage interests, net	0	0
Mortgage loans held for sale	2,170,200,000	1,630,800,000
Mortgage loans held for investment, net	0	0
Derivative financial instruments	72,000,000	49,000,000
Financial liabilities
Unsecured senior notes	0	0
Advance facilities	578,000,000	595,000,000
Warehouse facilities	3,050,000,000	2,349,000,000
Mortgage servicing rights financing liability	0	0
Excess spread financing	0	0
Derivative financial instruments at fair value	22,000,000	20,000,000
Recurring Fair Value Measurements \| Level 2 \| Participating Interest Financing
Financial liabilities
Other nonrecourse debt	0	0
Recurring Fair Value Measurements \| Level 2 \| Trust 2017-1
Financial liabilities
Other nonrecourse debt	0
Recurring Fair Value Measurements \| Level 2 \| Trust 2017-2
Financial liabilities
Other nonrecourse debt	0	0
Recurring Fair Value Measurements \| Level 2 \| Trust 2018-1
Financial liabilities
Other nonrecourse debt	0	0
Recurring Fair Value Measurements \| Level 2 \| Trust 2018-2
Financial liabilities
Other nonrecourse debt	0	0
Recurring Fair Value Measurements \| Level 2 \| Trust 2018-3
Financial liabilities
Other nonrecourse debt		0
Recurring Fair Value Measurements \| Level 2 \| Legacy Asset
Financial liabilities
Other nonrecourse debt	0	0
Recurring Fair Value Measurements \| Level 3
Financial assets
Cash and cash equivalents	0	0
Restricted cash	0	0
Advances and other receivables, net	1,147,000,000	1,194,000,000
Reverse mortgage interests, net	7,501,000,000	7,942,000,000
Mortgage loans held for sale	0	0
Mortgage loans held for investment, net	117,800,000	119,100,000
Derivative financial instruments	0	0
Financial liabilities
Unsecured senior notes	0	0
Advance facilities	0	0
Warehouse facilities	0	0
Mortgage servicing rights financing liability	33,700,000	31,700,000
Excess spread financing	1,309,000,000	1,184,000,000
Derivative financial instruments at fair value	0	0
Recurring Fair Value Measurements \| Level 3 \| Participating Interest Financing
Financial liabilities
Other nonrecourse debt	5,364,000,000	5,672,000,000
Recurring Fair Value Measurements \| Level 3 \| Trust 2017-1
Financial liabilities
Other nonrecourse debt	206,000,000
Recurring Fair Value Measurements \| Level 3 \| Trust 2017-2
Financial liabilities
Other nonrecourse debt	252,000,000	230,000,000
Recurring Fair Value Measurements \| Level 3 \| Trust 2018-1
Financial liabilities
Other nonrecourse debt	212,000,000	284,000,000
Recurring Fair Value Measurements \| Level 3 \| Trust 2018-2
Financial liabilities
Other nonrecourse debt	312,000,000	249,000,000
Recurring Fair Value Measurements \| Level 3 \| Trust 2018-3
Financial liabilities
Other nonrecourse debt		326,000,000
Recurring Fair Value Measurements \| Level 3 \| Legacy Asset
Financial liabilities
Other nonrecourse debt	$ 25,000,000	$ 28,000,000

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