Mortgage Loans - Loan Commitments and Related Derivatives Textual (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Interest rate lock commitments [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 1,800.0	$ 727.9
Best-efforts whole loan delivery commitments [Member]
Derivative [Line Items]
Derivative, Notional Amount	84.6	25.2
Hedging Instruments Related To IRLCs [Member]
Derivative [Line Items]
Derivative, Notional Amount	1,600.0	636.2
Loan Origination Commitments [Member]
Derivative [Line Items]
Derivative, Notional Amount	1,100.0	111.4
Fair Value, Inputs, Level 2 [Member] \| Fair Value, Recurring [Member] \| Interest rate lock commitments [Member]
Derivative [Line Items]
Interest Rate Derivative Instruments Not Designated as Hedging Instruments at Fair Value, Net	31.3	19.2
Fair Value, Inputs, Level 2 [Member] \| Fair Value, Recurring [Member] \| Best-efforts whole loan delivery commitments [Member]
Derivative [Line Items]
Interest Rate Derivative Instruments Not Designated as Hedging Instruments at Fair Value, Net		(1.0)
Fair Value, Inputs, Level 2 [Member] \| Fair Value, Recurring [Member] \| Hedging Instruments Related To IRLCs [Member]
Derivative [Line Items]
Interest Rate Derivative Instruments Not Designated as Hedging Instruments at Fair Value, Net	(16.2)	(4.1)
Fair Value, Inputs, Level 2 [Member] \| Fair Value, Recurring [Member] \| Loan Origination Commitments [Member]
Derivative [Line Items]
Interest Rate Derivative Instruments Not Designated as Hedging Instruments at Fair Value, Net	$ (5.3)	$ (0.5)

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
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-Paragraph 1A
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