Senior Notes - Principal Amounts Carrying Amounts and Fair Values of Senior Notes (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Debt Instrument [Line Items]
Carrying amounts	$ 750,000	$ 485,000
Fair value	800,750	514,121
Debt issuance costs	10,600	5,200
July 2010 Senior Notes
Debt Instrument [Line Items]
Carrying amounts	0	85,000
Fair value	0	86,121
May 2018 Senior Notes
Debt Instrument [Line Items]
Carrying amounts	400,000	400,000
Fair value	442,000	428,000
December 2019 Senior Notes
Debt Instrument [Line Items]
Carrying amounts	350,000	0
Fair value	$ 358,750	$ 0

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

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-Topic 944
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-Section S99
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