Fair Value Measurements Fair Value Measurements - Level 3 Rollforward (Details) - USD ($)	12 Months Ended
Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Mortgage loans held for sale	$ 2,386,000,000	$ 2,027,300,000
Mortgage servicing rights	$ 10,600,000	$ 4,100,000
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Revenues	Revenues
Mortgage hedging instruments and commitments	$ 330,200,000	$ 8,700,000
Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Purchases		0
Sales and Settlements		(3,900,000)
Principal Reductions		0
Net transfers to (out of) Level 3		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		0
Loans Receivable [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Purchases	0	0
Sales and Settlements	(3,900,000)	(3,300,000)
Principal Reductions	0	0
Net transfers to (out of) Level 3	7,000,000.0	(400,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(700,000)	300,000
Contractual Rights
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Purchases	35,300,000	24,700,000
Sales and Settlements	(27,600,000)	(37,500,000)
Principal Reductions	0	0
Net transfers to (out of) Level 3	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(1,200,000)	(200,000)
Level 3 [Member] \| Fair Value, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt securities collateralized by residential real estate		0	$ 3,900,000
Mortgage loans held for sale	14,100,000	11,700,000	15,100,000
Mortgage servicing rights	10,600,000	$ 4,100,000	$ 17,100,000
Measurement Input, Prepayment Rate [Member] \| Level 3 [Member] \| Fair Value, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Servicing Asset at Fair Value, Amount	0.12
Measurement Input, Discount Rate [Member] \| Level 3 [Member] \| Fair Value, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Servicing Asset at Fair Value, Amount	0.11
Measurement Input, Default Rate [Member] \| Level 3 [Member] \| Fair Value, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Servicing Asset at Fair Value, Amount	$ 0.08

Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as an asset.

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Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

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Indicates line item in statement in which net income is reported that includes gain (loss) from asset measured at fair value using unobservable input (level 3).

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Amount, after valuation allowance, of financing receivable held for sale and not part of disposal group. Excludes loan covered under loss sharing agreement and loan classified as investment in debt security.

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Amount of investments classified as other.

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Aggregate amount of servicing assets that are subsequently measured at fair value and servicing assets that are subsequently measured using the amortization method.

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Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

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