Fair Value Measurements - Fair Value by Balance Sheet Line Item (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Financial assets
Restricted cash	$ 271.0	$ 319.0
Reverse mortgage interests, net	6,662.0	7,934.0
Mortgage loans held for sale	4,267.0	1,631.0
Mortgage loans held for investment	0.0	119.0
Financial liabilities
Unsecured senior notes	2,464.0	2,459.0
Advance facilities	513.0	595.0
Warehouse facilities	4,802.0	2,349.0
Mortgage servicing rights financing liability	47.0	32.0
Excess spread financing	1,281.0	1,184.0
Other nonrecourse debt	5,533.0	6,795.0
Participating interest financing
Financial liabilities
Other nonrecourse debt	4,581.0	5,607.0
Trust 2017-2
Financial liabilities
Other nonrecourse debt	0.0	231.0
Trust 2018-1
Financial liabilities
Other nonrecourse debt	201.0	284.0
Trust 2018-2
Financial liabilities
Other nonrecourse debt	161.0	250.0
Trust 2018-3
Financial liabilities
Other nonrecourse debt	239.0	326.0
Trust 2019-1
Financial liabilities
Other nonrecourse debt	339.0	0.0
Nonrecourse debt - legacy assets
Financial liabilities
Other nonrecourse debt	0.0	29.0
Recurring Fair Value Measurements
Financial assets
Cash and cash equivalents	371.0	242.0
Restricted cash	271.0	319.0
Advances and other receivables, net	967.0	1,194.0
Reverse mortgage interests, net	6,662.0	7,934.0
Mortgage loans held for sale	4,267.2	1,630.8
Mortgage loans held for investment		119.1
Derivative financial instruments	170.0	49.0
Financial liabilities
Unsecured senior notes	2,464.0	2,459.0
Advance facilities	513.0	595.0
Warehouse facilities	4,802.0	2,349.0
Mortgage servicing rights financing liability	46.9	31.7
Excess spread financing	1,281.0	1,184.0
Derivative financial instruments at fair value	19.0	20.0
Recurring Fair Value Measurements \| Participating interest financing
Financial liabilities
Other nonrecourse debt	4,593.0	5,675.0
Recurring Fair Value Measurements \| Trust 2017-2
Financial liabilities
Other nonrecourse debt		231.0
Recurring Fair Value Measurements \| Trust 2018-1
Financial liabilities
Other nonrecourse debt	201.0	284.0
Recurring Fair Value Measurements \| Trust 2018-2
Financial liabilities
Other nonrecourse debt	161.0	250.0
Recurring Fair Value Measurements \| Trust 2018-3
Financial liabilities
Other nonrecourse debt	239.0	326.0
Recurring Fair Value Measurements \| Trust 2019-1
Financial liabilities
Other nonrecourse debt	339.0
Recurring Fair Value Measurements \| Nonrecourse debt - legacy assets
Financial liabilities
Other nonrecourse debt		29.0
Recurring Fair Value Measurements \| Level 1
Financial assets
Cash and cash equivalents	371.0	242.0
Restricted cash	271.0	319.0
Advances and other receivables, net	0.0	0.0
Reverse mortgage interests, net	0.0	0.0
Mortgage loans held for sale	0.0	0.0
Mortgage loans held for investment		0.0
Derivative financial instruments	0.0	0.0
Financial liabilities
Unsecured senior notes	2,592.0	2,451.0
Advance facilities	0.0	0.0
Warehouse facilities	0.0	0.0
Mortgage servicing rights financing liability	0.0	0.0
Excess spread financing	0.0	0.0
Derivative financial instruments at fair value	0.0	0.0
Recurring Fair Value Measurements \| Level 1 \| Participating interest financing
Financial liabilities
Other nonrecourse debt	0.0	0.0
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	0.0
Recurring Fair Value Measurements \| Level 1 \| Trust 2018-2
Financial liabilities
Other nonrecourse debt	0.0	0.0
Recurring Fair Value Measurements \| Level 1 \| Trust 2018-3
Financial liabilities
Other nonrecourse debt	0.0	0.0
Recurring Fair Value Measurements \| Level 1 \| Trust 2019-1
Financial liabilities
Other nonrecourse debt	0.0
Recurring Fair Value Measurements \| Level 1 \| Nonrecourse debt - legacy assets
Financial liabilities
Other nonrecourse debt		0.0
Recurring Fair Value Measurements \| Level 2
Financial assets
Cash and cash equivalents	0.0	0.0
Restricted cash	0.0	0.0
Advances and other receivables, net	0.0	0.0
Reverse mortgage interests, net	0.0	0.0
Mortgage loans held for sale	4,267.2	1,630.8
Mortgage loans held for investment		0.0
Derivative financial instruments	170.0	49.0
Financial liabilities
Unsecured senior notes	0.0	0.0
Advance facilities	513.0	595.0
Warehouse facilities	4,802.0	2,349.0
Mortgage servicing rights financing liability	0.0	0.0
Excess spread financing	0.0	0.0
Derivative financial instruments at fair value	19.0	20.0
Recurring Fair Value Measurements \| Level 2 \| Participating interest financing
Financial liabilities
Other nonrecourse debt	0.0	0.0
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	0.0
Recurring Fair Value Measurements \| Level 2 \| Trust 2018-2
Financial liabilities
Other nonrecourse debt	0.0	0.0
Recurring Fair Value Measurements \| Level 2 \| Trust 2018-3
Financial liabilities
Other nonrecourse debt	0.0	0.0
Recurring Fair Value Measurements \| Level 2 \| Trust 2019-1
Financial liabilities
Other nonrecourse debt	0.0
Recurring Fair Value Measurements \| Level 2 \| Nonrecourse debt - legacy assets
Financial liabilities
Other nonrecourse debt		0.0
Recurring Fair Value Measurements \| Level 3
Financial assets
Cash and cash equivalents	0.0	0.0
Restricted cash	0.0	0.0
Advances and other receivables, net	967.0	1,194.0
Reverse mortgage interests, net	6,726.0	7,934.0
Mortgage loans held for sale	0.0	0.0
Mortgage loans held for investment		119.1
Derivative financial instruments	0.0	0.0
Financial liabilities
Unsecured senior notes	0.0	0.0
Advance facilities	0.0	0.0
Warehouse facilities	0.0	0.0
Mortgage servicing rights financing liability	46.9	31.7
Excess spread financing	1,281.0	1,184.0
Derivative financial instruments at fair value	0.0	0.0
Recurring Fair Value Measurements \| Level 3 \| Participating interest financing
Financial liabilities
Other nonrecourse debt	4,590.0	5,672.0
Recurring Fair Value Measurements \| Level 3 \| Trust 2017-2
Financial liabilities
Other nonrecourse debt		230.0
Recurring Fair Value Measurements \| Level 3 \| Trust 2018-1
Financial liabilities
Other nonrecourse debt	201.0	284.0
Recurring Fair Value Measurements \| Level 3 \| Trust 2018-2
Financial liabilities
Other nonrecourse debt	161.0	249.0
Recurring Fair Value Measurements \| Level 3 \| Trust 2018-3
Financial liabilities
Other nonrecourse debt	239.0	326.0
Recurring Fair Value Measurements \| Level 3 \| Trust 2019-1
Financial liabilities
Other nonrecourse debt	$ 339.0
Recurring Fair Value Measurements \| Level 3 \| Nonrecourse debt - legacy assets
Financial liabilities
Other nonrecourse debt		$ 28.0

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