Derivative Financial Instruments - Outstanding Balances (Details) - USD ($) $ in Millions	5 Months Ended	7 Months Ended	12 Months Ended
Dec. 31, 2018	Jul. 31, 2018	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Derivative asset, fair value	$ 49.0		$ 153.0
Derivative instrument, fair value (less than)	0.1		0.1
Assets
Derivatives, Fair Value [Line Items]
Derivative asset, notional amount	2,020.0		9,058.0
Derivative asset, fair value	49.4		153.4
Recorded gains / (losses)	(14.9)	$ (0.2)	90.1
Liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, notional amount	2,735.0		6,595.0
Derivative liability, fair value	19.7		14.9
Recorded gains / (losses)	17.1	(2.2)	(9.5)
Loan sale commitments \| Assets
Derivatives, Fair Value [Line Items]
Derivative asset, notional amount	319.0		1,202.0
Derivative asset, fair value	13.5		32.3
Recorded gains / (losses)	2.8	10.5	6.5
IRLCs \| Assets
Derivatives, Fair Value [Line Items]
Derivative asset, notional amount	1,301.0		4,838.0
Derivative asset, fair value	47.6		134.7
Recorded gains / (losses)	(12.1)	0.4	75.0
IRLCs \| Liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, notional amount	0.0		12.0
Derivative liability, fair value	0.0		0.0
Recorded gains / (losses)	0.0	0.0	0.0
LPCs \| Assets
Derivatives, Fair Value [Line Items]
Derivative asset, notional amount	215.0		1,094.0
Derivative asset, fair value	1.7		12.2
Recorded gains / (losses)	0.4	0.3	10.5
LPCs \| Liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, notional amount	90.0		540.0
Derivative liability, fair value	0.4		2.6
Recorded gains / (losses)	(0.2)	0.1	2.1
Forward sales of MBS \| Assets
Derivatives, Fair Value [Line Items]
Derivative asset, notional amount	485.0		3,120.0
Derivative asset, fair value	0.1		6.5
Recorded gains / (losses)	(3.1)	0.9	4.6
Forward sales of MBS \| Liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, notional amount	2,639.0		6,036.0
Derivative liability, fair value	19.3		12.3
Recorded gains / (losses)	17.4	(1.0)	(11.6)
Treasury futures \| Assets
Derivatives, Fair Value [Line Items]
Derivative asset, notional amount	0.0
Derivative asset, fair value	0.0
Recorded gains / (losses)	(0.1)	(1.8)
Treasury futures \| Liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, notional amount	0.0
Derivative liability, fair value	0.0
Recorded gains / (losses)	(0.1)	(1.3)
Eurodollar futures \| Assets
Derivatives, Fair Value [Line Items]
Derivative asset, notional amount	19.0		6.0
Derivative asset, fair value	0.0		0.0
Recorded gains / (losses)	0.0	0.0	0.0
Eurodollar futures \| Liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, notional amount	6.0		7.0
Derivative liability, fair value	0.0		0.0
Recorded gains / (losses)	$ 0.0	$ 0.0	$ 0.0

Nominal or face amount used to calculate payments on the derivative asset.

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