Fair Value Measurements - Measured on a Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Assets
Mortgage loans held for sale	$ 3,422.0	$ 1,631.0
Mortgage loans held for investment	114.0	119.0
Mortgage servicing rights	3,505.0	3,665.0
Liabilities
Mortgage servicing rights financing	43.0	32.0
Derivative instruments at fair value, less than	0.1	0.1	$ 0.1
Recurring Fair Value Measurements
Assets
Mortgage loans held for sale	3,421.6	1,630.8
Mortgage loans held for investment	114.3	119.1
Mortgage servicing rights	3,504.5	3,665.4
Derivative financial instruments	128.0	49.0
Total assets	7,168.2	5,464.7
Liabilities
Mortgage servicing rights financing	42.6	31.7
Excess spread financing	1,429.4	1,184.4
Total liabilities	1,503.6	1,235.8
Recurring Fair Value Measurements \| IRLCs
Assets
Derivative financial instruments	110.2	47.6
Liabilities
Derivative financial instruments	0.0
Recurring Fair Value Measurements \| Forward sales of MBS
Assets
Derivative financial instruments	1.1	0.1
Liabilities
Derivative financial instruments	30.3	19.3
Recurring Fair Value Measurements \| LPCs
Assets
Derivative financial instruments	16.5	1.7
Liabilities
Derivative financial instruments	1.3	0.4
Recurring Fair Value Measurements \| Eurodollar futures
Assets
Derivative financial instruments	0.0	0.0
Liabilities
Derivative financial instruments	0.0	0.0
Recurring Fair Value Measurements \| Level 1
Assets
Mortgage loans held for sale	0.0	0.0
Mortgage loans held for investment	0.0	0.0
Mortgage servicing rights	0.0	0.0
Derivative financial instruments	0.0	0.0
Total assets	0.0	0.0
Liabilities
Mortgage servicing rights financing	0.0	0.0
Excess spread financing	0.0	0.0
Total liabilities	0.0	0.0
Recurring Fair Value Measurements \| Level 1 \| IRLCs
Assets
Derivative financial instruments	0.0	0.0
Liabilities
Derivative financial instruments	0.0
Recurring Fair Value Measurements \| Level 1 \| Forward sales of MBS
Assets
Derivative financial instruments	0.0	0.0
Liabilities
Derivative financial instruments	0.0	0.0
Recurring Fair Value Measurements \| Level 1 \| LPCs
Assets
Derivative financial instruments	0.0	0.0
Liabilities
Derivative financial instruments	0.0	0.0
Recurring Fair Value Measurements \| Level 1 \| Eurodollar futures
Assets
Derivative financial instruments	0.0	0.0
Liabilities
Derivative financial instruments	0.0	0.0
Recurring Fair Value Measurements \| Level 2
Assets
Mortgage loans held for sale	3,421.6	1,630.8
Mortgage loans held for investment	0.0	0.0
Mortgage servicing rights	0.0	0.0
Derivative financial instruments	128.0	49.0
Total assets	3,549.4	1,680.2
Liabilities
Mortgage servicing rights financing	0.0	0.0
Excess spread financing	0.0	0.0
Total liabilities	31.6	19.7
Recurring Fair Value Measurements \| Level 2 \| IRLCs
Assets
Derivative financial instruments	110.2	47.6
Liabilities
Derivative financial instruments	0.0
Recurring Fair Value Measurements \| Level 2 \| Forward sales of MBS
Assets
Derivative financial instruments	1.1	0.1
Liabilities
Derivative financial instruments	30.3	19.3
Recurring Fair Value Measurements \| Level 2 \| LPCs
Assets
Derivative financial instruments	16.5	1.7
Liabilities
Derivative financial instruments	1.3	0.4
Recurring Fair Value Measurements \| Level 2 \| Eurodollar futures
Assets
Derivative financial instruments	0.0	0.0
Liabilities
Derivative financial instruments	0.0	0.0
Recurring Fair Value Measurements \| Level 3
Assets
Mortgage loans held for sale	0.0	0.0
Mortgage loans held for investment	114.3	119.1
Mortgage servicing rights	3,504.5	3,665.4
Derivative financial instruments	0.0	0.0
Total assets	3,618.8	3,784.5
Liabilities
Mortgage servicing rights financing	42.6	31.7
Excess spread financing	1,429.4	1,184.4
Total liabilities	1,472.0	1,216.1
Recurring Fair Value Measurements \| Level 3 \| IRLCs
Assets
Derivative financial instruments	0.0	0.0
Liabilities
Derivative financial instruments	0.0
Recurring Fair Value Measurements \| Level 3 \| Forward sales of MBS
Assets
Derivative financial instruments	0.0	0.0
Liabilities
Derivative financial instruments	0.0	0.0
Recurring Fair Value Measurements \| Level 3 \| LPCs
Assets
Derivative financial instruments	0.0	0.0
Liabilities
Derivative financial instruments	0.0	0.0
Recurring Fair Value Measurements \| Level 3 \| Eurodollar futures
Assets
Derivative financial instruments	0.0	0.0
Liabilities
Derivative financial instruments	$ 0.0	$ 0.0

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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