Fair Value Measurements - Fair Value by Balance Sheet Line Item (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Financial assets
Restricted cash	$ 304.0	$ 319.0
Reverse mortgage interests, net	7,110.0	7,934.0
Mortgage loans held for sale	3,422.0	1,631.0
Mortgage loans held for investment, net	114.0	119.0
Financial liabilities
Unsecured senior notes	2,462.0	2,459.0
Advance facilities	567.0	595.0
Warehouse facilities	4,045.0	2,349.0
Mortgage servicing rights financing liability	43.0	32.0
Excess spread financing	1,429.0	1,184.0
Other nonrecourse debt	5,985.0	6,795.0
Participating Interest Financing
Financial liabilities
Other nonrecourse debt	4,861.0	5,607.0
Trust 2017-2
Financial liabilities
Other nonrecourse debt	0.0	231.0
Trust 2018-1
Financial liabilities
Other nonrecourse debt	224.0	284.0
Trust 2018-2
Financial liabilities
Other nonrecourse debt	182.0	250.0
Trust 2018-3
Financial liabilities
Other nonrecourse debt	272.0	326.0
Trust 2019-1
Financial liabilities
Other nonrecourse debt	398.0	0.0
Legacy Asset
Financial liabilities
Other nonrecourse debt	22.0	29.0
Recurring Fair Value Measurements
Financial assets
Cash and cash equivalents	245.0	242.0
Restricted cash	304.0	319.0
Advances and other receivables, net	1,000.0	1,194.0
Reverse mortgage interests, net	7,110.0	7,934.0
Mortgage loans held for sale	3,421.6	1,630.8
Mortgage loans held for investment, net	114.3	119.1
Derivative financial instruments	128.0	49.0
Financial liabilities
Unsecured senior notes	2,462.0	2,459.0
Advance facilities	567.0	595.0
Warehouse facilities	4,045.0	2,349.0
Mortgage servicing rights financing liability	42.6	31.7
Excess spread financing	1,429.0	1,184.0
Derivative financial instruments at fair value	32.0	20.0
Recurring Fair Value Measurements \| Participating Interest Financing
Financial liabilities
Other nonrecourse debt	4,887.0	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	224.0	284.0
Recurring Fair Value Measurements \| Trust 2018-2
Financial liabilities
Other nonrecourse debt	182.0	250.0
Recurring Fair Value Measurements \| Trust 2018-3
Financial liabilities
Other nonrecourse debt	272.0	326.0
Recurring Fair Value Measurements \| Trust 2019-1
Financial liabilities
Other nonrecourse debt	398.0
Recurring Fair Value Measurements \| Legacy Asset
Financial liabilities
Other nonrecourse debt	22.0	29.0
Recurring Fair Value Measurements \| Level 1
Financial assets
Cash and cash equivalents	245.0	242.0
Restricted cash	304.0	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, net	0.0	0.0
Derivative financial instruments	0.0	0.0
Financial liabilities
Unsecured senior notes	2,529.0	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 \| Legacy Asset
Financial liabilities
Other nonrecourse debt	0.0	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	3,421.6	1,630.8
Mortgage loans held for investment, net	0.0	0.0
Derivative financial instruments	128.0	49.0
Financial liabilities
Unsecured senior notes	0.0	0.0
Advance facilities	567.0	595.0
Warehouse facilities	4,045.0	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	32.0	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 \| Legacy Asset
Financial liabilities
Other nonrecourse debt	0.0	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	1,000.0	1,194.0
Reverse mortgage interests, net	7,176.0	7,942.0
Mortgage loans held for sale	0.0	0.0
Mortgage loans held for investment, net	114.3	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	42.6	31.7
Excess spread financing	1,429.0	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	4,886.0	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	224.0	284.0
Recurring Fair Value Measurements \| Level 3 \| Trust 2018-2
Financial liabilities
Other nonrecourse debt	182.0	249.0
Recurring Fair Value Measurements \| Level 3 \| Trust 2018-3
Financial liabilities
Other nonrecourse debt	272.0	326.0
Recurring Fair Value Measurements \| Level 3 \| Trust 2019-1
Financial liabilities
Other nonrecourse debt	398.0
Recurring Fair Value Measurements \| Level 3 \| Legacy Asset
Financial liabilities
Other nonrecourse debt	$ 22.0	$ 28.0

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