Senior Notes - Principal Amounts Carrying Amounts and Fair Values of Senior Notes (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
Carrying amounts	$ 513,000	$ 244,000
Fair value	518,413	253,356
Debt issuance costs, net	(6,100)	(200)
May 2008 Senior Notes
Debt Instrument [Line Items]
Carrying amounts	0	131,000
Fair value	0	134,250
July 2010 Senior Notes
Debt Instrument [Line Items]
Carrying amounts	113,000	113,000
Fair value	114,413	119,106
May 2018 Senior Notes
Debt Instrument [Line Items]
Carrying amounts	400,000	0
Fair value	$ 404,000	$ 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
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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.

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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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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