Fair Value Measurements - Assets and Liabilities Not Reflected at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group	$ 1,379.4	$ 1,072.0
Cash and cash equivalents, Carrying value	1,522.8	1,494.3
Restricted Cash and Cash Equivalents	14.5	19.7
Notes payable	4,306.6	3,399.4
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Fair value	1,522.8	1,494.3
Restricted cash, Fair value	14.5	19.7
Notes payable, Fair value	4,265.7	3,525.8
Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Fair value	1,522.8	1,494.3
Restricted cash, Fair value	14.5	19.7
Notes payable, Fair value	0.0	0.0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Fair value	0.0	0.0
Restricted cash, Fair value	0.0	0.0
Notes payable, Fair value	2,541.2	2,533.9
Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Fair value	0.0	0.0
Restricted cash, Fair value	0.0	0.0
Notes payable, Fair value	1,724.5	991.9
Fair Value, Nonrecurring [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group	0.0	0.0
Loans Receivable, Fair Value Disclosure	0.0	0.0
Fair Value, Nonrecurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Inventories [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Inventories	0.0	0.0
Fair Value, Nonrecurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Disposal Group, Held-for-sale, Not Discontinued Operations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Inventories	0.0	0.0
Fair Value, Nonrecurring [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group	1.3	2.7
Loans Receivable, Fair Value Disclosure	1.1	1.8
Fair Value, Nonrecurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Inventories [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Inventories	0.0	4.5
Fair Value, Nonrecurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Disposal Group, Held-for-sale, Not Discontinued Operations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Inventories	$ 12.3	$ 0.0

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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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

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Amount, after allowance for bad debt, of loan receivables held-for-sale that are not part of a disposal group. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities.

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Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

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Fair value portion of other assets.

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Amount of cash and cash equivalents restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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